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  • Articles  (10,835)
  • American Institute of Physics (AIP)  (10,835)
  • 1990-1994  (10,835)
  • Chemistry and Pharmacology  (10,835)
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  • 1
    Electronic Resource
    Electronic Resource
    Semiclassical surface-hopping approximations for the calculation of solvent-induced vibrational-relaxation rate constants (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7520-7527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Approximate schemes for the calculation of the rates of transitions between vibrational states of a molecule due to the interactions with a solvent are devised based on a rigorous, general semiclassical surface-hopping formalism developed earlier. The formal framework is based on an adiabatic separation of time scales between the fast molecular vibrations and the relatively slow bath motions. (The bath is composed of the solvent degrees of freedom plus all the molecular degrees of freedom other than vibrations.) As a result, the dynamics of the system are described in terms of bath motions occurring on adiabatic vibrational-energy surfaces, which are coupled by a nonadiabatic interaction. The time-dependent vibrational transition probability is evaluated by propagating the canonical density of the system, with the molecule in the initial adiabatic vibrational state, forward in time, and then projecting it onto the final adiabatic vibrational state of interest. The temporal evolution of the density is carried out with a semiclassical surface-hopping propagator, in which the motion of the bath on an adiabatic vibrational surface is described in terms of the familiar (adiabatic) semiclassical propagator, while transitions are accounted for in terms of instantaneous hops of the bath paths between the adiabatic vibrational surfaces involved, with an integration over all possible hopping points.Energy is conserved in the hops, and the only component of momentum that changes is the one along the nonadiabatic coupling vector. When the nonadiabatic interaction is taken into account to first order, the transition probability is predicted to become linear in the long-time limit. Various methods for extracting the relaxation rate constant in this limit are presented, and a simple model system with a one-dimensional bath is employed to compare their practical efficiency for finite time. In addition, this system is used to numerically demonstrate that local approximations for the adiabatic vibrational surfaces and the nonadiabatic coupling yield accurate results, with great reduction of the amount of computation time. Since a local approximation for the vibrational surfaces makes an N-dimensional problem separable into N effectively one-dimensional ones, this treatment is seen to be more generally applicable to realistic systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468245
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  • 2
    Electronic Resource
    Electronic Resource
    Theoretical study of Ca(4s5p 1P)→Ca(4s5p 3P) energy transfer in collisions with He. Initial and final state alignment (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7554-7565 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we present the results of close-coupling calculations of the cross sections for Ca(4s5p 1P)→Ca(4s5p 3P) energy transfer in collisions with He, based on new potential energy curves. Particular attention is devoted to the simulation of the recent experiment of Smith and co-workers [J. Chem. Phys. 96, 8212 (1992)], in which, for the 1P→3P2 transfer both initial and final alignment are controlled with respect to the initial relative velocity vector of the two partners Vrel. The calculated polarization ratios (σ⊥/σ(parallel)), defined as the ratio of the cross sections for the 1P→3P transfer summed over final levels for initial alignment of the 5p orbital perpendicular and parallel to Vrel are in good agreement with the experimental results. The theoretical cross sections for the 1P→3P2 transfer determined in the so-called collision frame (σj1m1m1→j2m2m2), where the axis of quantization is taken along Vrel are in good agreement with the experiment in the case of initial perpendicular excitation, but show quantitative and qualitative disagreement in the case of initial parallel excitation. We find that relative populations into the final m2 levels depend strongly on the initial orientation of the 5p orbital with respect to the initial collision plane. However, we show, that in the case where the 5p orbital lies in the initial collision plane, the relative populations in the final m2 components of the 3P2 state depend exclusively on interactions among the exit channels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468250
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  • 3
    Electronic Resource
    Electronic Resource
    On the energy dependence of the steric effect in atom–molecule reactive scattering. I. A quasiclassical approach (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7592-7602 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Experimental studies have shown that the steric effect in chemical reactions can decrease (e.g., for Ba+N2O→BaO*+N2) or increase [e.g., for Ca(1D2)+CH3F→CaF*+CH3] with increasing translational energy. Decreasing (negative) energy dependences have successfully been modeled with the angle dependent line of centers model. We present a classical model in which a positive energy dependence of the steric effect is explained by an isotropic, attractive long range potential. In this "trapping'' model we assume the reaction—apart from a cone of nonreaction at one side of the molecule—to be barrierless. This model shows that a positive energy dependence of the steric effect is not indicative of reorientation of the molecule, as has been suggested in the literature. Rather, the positive or negative energy dependence of the steric effect is shown to correlate with the absence or presence of a barrier to reaction and an attractive or repulsive long range potential. For the reorientation effects which occur in the case of anisotropic potentials, we consider the application of the standard quasiclassical trajectory (QCT) method and we introduce a modified QCT method. We argue that the latter is more suitable for the computation of the orientation dependent reactive cross section.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468253
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  • 4
    Electronic Resource
    Electronic Resource
    Hydrated proton clusters and solvent effects on the proton transfer barrier: A density functional study (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7633-7642 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The density functional calculations using the Perdew nonlocal corrections to exchange and correlation have been carried out for a sequence of hydrated proton clusters. The optimized structures were obtained up to H13O+6. It is found that H3O+ is indeed the central unit in all the lowest energy structures we found. Our results support the argument that the structure with a four-coordinate first solvation shell is very unlikely in small hydrated proton clusters. The density functional calculations with the Perdew nonlocal corrections to exchange and correlation give somewhat shorter hydrogen bond lengths, but slightly longer chemical bond lengths as compared with the post-Hartree–Fock calculations. The harmonic vibrational frequencies and IR intensities of various vibrational modes have been generated for all the structures optimized. Results for small clusters are compared with the high resolution experimental spectroscopy studies of Yeh et al. and Begemann et al. Results for larger clusters are used to interpret the low resolution spectra of Schwartz. Very good accord with experimental results is obtained. The solvent effects on proton transfer energy barriers in clusters have been studied by designing a few model systems. The barrier is found to be very sensitive to the solvent configurations. When the solvent water is replaced by the classical partial charge model, a significant change of the barrier is observed, indicating that a quantitative treatment will ultimately require a good pseudopotential to properly account for the quantum nature of the solvent. A combined density functional and molecular dynamics simulation was used to calculate the proton transfer energy and free energy barrier in aqueous solution. The barrier is found to be 3 kcal/mol higher than in gas phase. Very large solvent fluctuation is observed which may have a significant influence on the reaction rate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468256
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  • 5
    Electronic Resource
    Electronic Resource
    Low-temperature proton transport in clathrates (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7672-7682 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The impedance spectra are measured for protonated and deuterated clathrates, HClO4⋅5.5H2O and DClO4⋅5.5D2O, between 10 and 300 K. The conductance is investigated between 80 K and room temperature and the dielectric constant between 10 and 120 K. The data show deviation from the Arrhenius behavior of conductance in the low-temperature regime. A description of proton conductivity is developed on the basis of quantum theory of an elementary act of proton tunneling between donor–acceptor sites interacting with environmental fluctuations. Several models of the elementary act are considered. The mechanism, most consistent with the obtained data, incorporates—strong coupling of the proton with local vibrational modes of the closest environment and system diabatic transitions along these vibrational "coordinates''—fluctuations of the tunneling barrier for the proton. At low temperatures the motion along the vibrational coordinates is no longer purely classical and the slow mode tunneling takes place. The latter gives rise to a curvature in the Arrhenius plots apprehended as a decrease of the apparent activation energy at lower temperatures. The observed isotope effect is in line with the lower deuteron tunneling probability due to doubling the mass, though other parameters may also be affected by deuteration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468260
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  • 6
    Electronic Resource
    Electronic Resource
    Optical potential coupled to discrete variable representation for calculations of quasibound states: Application to the CO(B 1Σ+–D'1 Σ+) predissociating interaction (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7580-7591 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The optical potential method initially proposed by Jolicard and Austin in the context of the stabilization method is reviewed here and used with the accurate and the efficient discrete variable representation method to obtain the energies and the widths (respectively, the real and the imaginary part of the resonance energies) of rovibrational predissociated states of diatomic molecules. In this method the resonances for an n coupled states problem are obtained by a direct diagonalization of the Hamiltonian matrix in the diabatic representation. This Hamiltonian matrix is directly evaluated in the discrete variable representation using the Fourier grid Hamiltonian method proposed by Marston and Balint-Kurti. In this approach, two optical potentials are tested and used here to impose the asymptotic behaviors of the boundary conditions which are compatible with the resonance states. The method is exemplified for the B 1Σ+–D'1 Σ+ Rydberg–valence predissociating interaction in the CO molecule.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468252
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  • 7
    Electronic Resource
    Electronic Resource
    A self-adaptive multilevel finite element method for the stationary Schrödinger equation in three space dimensions (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7643-7650 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An error controlled finite element method (FEM) for solving stationary Schrödinger equations in three space dimensions is proposed. The method is based on an adaptive space discretization into tetrahedra and local polynomial basis functions of order p=1–5 defined on these tetrahedra. According to a local error estimator, the triangulation is automatically adapted to the solution. Numerical results for standard problems appearing in vibrational motion and molecular structure calculations are presented and discussed. Relative precisions better than 1e-8 are obtained. For equilateral H++3, the adaptive FEM turns out to be superior to global basis set expansions in the literature. Our precise FEM results exclude in a definite manner the stability or metastability of equilateral H++3 in its ground state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468257
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  • 8
    Electronic Resource
    Electronic Resource
    Pressure dependence of magnetic coupling in ionic solids from ab initio cluster model calculations (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7683-7685 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dependence of the magnetic coupling constant, J, with the pressure has been studied by an ab initio cluster model approach in a typical ionic solid such as KNiF3. By computing J at different values of the lattice parameter R, we predict a power law of the form ||J||≈R−n with 10.5〈n〈13.3 in good agreement with the suggested experimental value. This behavior is found even for the simplest Ni2F cluster model described by a complete active space configuration interaction wave function in which the active orbitals correspond to the open shell of each Ni2+ cation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468261
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  • 9
    Electronic Resource
    Electronic Resource
    Calculation of rovibronic structures in the lowest nine excited 1Σ+g+1Πg+1Δg states of H2, D2, and T2 (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7692-7706 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have calculated the rovibronic structures which arise from the simultaneous radial and angular couplings among the lowest nine excited 1Σ+g, 1Πg, and 1Δg states in H2, D2, and T2. Using ab initio potential curves, adiabatic corrections, and nonadiabatic coupling functions wherever possible, we have solved the coupled vibronic equations for all bound states below the 1s+2l dissociation limit up to J=5 in H2 and D2, and up to J=7 in T2. The good agreement of the bulk of the data with spectroscopic results enables us to eliminate spurious assignments in old spectroscopic work (e.g., all old assignments involving the 4s O 1Σ+g state are spurious), and to make numerous new assignments. We present updated tables of spectroscopic term values for H2 and D2. The absolute energy errors of the ab initio Born–Oppenheimer energies around the minima of the potential curves appear to be at most of the order of the radiative energy corrections ((approximately-equal-to)0.1 cm−1) in the states 1σg2sσg E and (1σu)2 F, less than 1 cm−1 in the states 3dσ G, 3s H, and 3dπ I, less than 2 cm−1 in the 4s O state, and approximately 4 cm−1 in the 4dσ P state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468263
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  • 10
    Electronic Resource
    Electronic Resource
    Computer simulations of cesium–water clusters: Do ion–water clusters form gas-phase clathrates? (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7873-7881 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and energetics of cesium ion–water clusters have been investigated using classical molecular dynamics computer simulations and a polarizable interaction model. Recent experiments by Selinger and Castleman [J. Phys. Chem. 95, 8442 (1991)] indicate that the mass-spectral distributions for these clusters exhibit "magic number'' oscillations at temperatures below approximately 160 K. The observed behavior of this and related charged clusters is commonly attributed to the formation of clathratelike cage structures around a central ionic species. The relationship between the structural and energetic properties of cesium ion–water clusters is reported here as a function of temperature for clusters with between 18 and 22 water molecules. The clusters exhibit solidlike dynamical behavior at kinetic temperatures below about 170 K, and liquidlike behavior at higher temperatures. A thorough analysis of energy minimized (0 K) structures indicates that the most stable clusters consist of water cages surrounding the cesium ion. These cages are related to the proposed clathratelike structures but contain additional 4- and 6-membered water rings and fewer 5-membered rings. The calculated global energy minima exhibit an energetic alternation with cluster size that is consistent with the experimentally observed mass-spectral distributions. In contrast, in the liquidlike regime there are only minor variations in calculated structural and dynamical properties as a function of cluster size. In addition, there is no statistically significant size dependence for the cluster binding energies in the liquidlike regime that might be correlated with experimental data. These results suggest that magic number stability in ion–water clusters may occur only at "low'' energies in the solidlike cluster regime.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468213
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  • 11
    Electronic Resource
    Electronic Resource
    Multibody distribution function contributions to the heat capacity for the truncated Lennard-Jones fluid (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7934-7938 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have performed a series of standard NVT Monte Carlo simulations in which we have calculated the configurational heat capacity, Cv, of a truncated Lennard-Jones model fluid by the fluctuation theorem. As we found in a previous investigation, the heat capacity exhibited two extrema as a function of density on sufficiently low temperature isotherms. During the course of these runs, we have decomposed the heat capacity into the sum of quantities, Cv(2), Cv(3), and Cv(4), which are averages over two, three, and four body distribution functions. We analyze combinations of the three contributions to gain insight into the nature of the extrema. In particular, we find that the sum Cv(2)+Cv(3) has a maximum and a minimum at all temperatures studied. The magnitude and slope of Cv(2)+Cv(3) compared with Cv(4) determines whether or not Cv itself has maxima and minima.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468220
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  • 12
    Electronic Resource
    Electronic Resource
    Water near a planar interface: Atom-based integral equation theory (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7925-7933 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and properties of an atom-based model of water next to a planar interface are solved using an approximate integral equation theory. The input to the calculations is the structure of the bulk water in the form of the direct correlation functions. Predictions from the theory include the oxygen and hydrogen density profiles perpendicular to the interface, the mean electrostatic potential, the potential of zero charge, and the differential capacitance. The predicted structure is relatively insensitive to both the surface potential and the details of the short-range wall–water potentials, and exhibits a layered structure which extends approximately 15 A(ring) into the liquid. For our initial choice of the short-range wall–water potential, we predict a value of −32 mV for the potential of zero charge and a differential capacitance of 4.89 μF cm−2 for pure water at a planar interface. The capacitance is apparently independent of the surface charge density and the surface potential.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468219
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  • 13
    Electronic Resource
    Electronic Resource
    Near ultraviolet photolysis of methanethiol studied by H atom photofragment translational spectroscopy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7538-7547 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The technique of H Rydberg atom photofragment translational spectroscopy has been applied to a high resolution study of the primary photochemistry of methanethiol (CH3SH) following excitation at a wide range of wavelengths in the near ultraviolet. In accord with previous studies of this molecule, excitation within its first (1 1A‘−X˜ 1A') absorption continuum is shown to result in S–H bond fission. Spectral analysis yields a refined value for the bond dissociation energy: D00(CH3S–H)=30 250±100 cm−1. The resulting CH3S(X˜) fragments are deduced to carry only modest vibrational excitation, distributed specifically in the ν3 (C–S stretching) mode and in one other mode having a wave number of ∼1040 cm−1. We associate this latter mode with bending of the CH3 moiety in the plane containing the C and S nuclei and the lobe of the unpaired electron which was originally involved in the S–H bond. Decreasing the excitation wavelength (while remaining within the first absorption continuum) results in an increase in both the vibrational and rotational excitation of the CH3S(X˜) fragments, but a decrease in the relative yield of the upper (2E1/2) spin–orbit component. Excitation at still shorter excitation wavelengths accesses the second (2 1A‘−X˜ 1A') absorption band of CH3SH. The CH3S fragments resulting from S–H bond fission at these excitation wavelengths are observed to carry very much higher levels of vibrational excitation in the above two modes. The observation of H atoms attributable to secondary photolysis of SH(X) fragments indicates increased competition from the alternative C–S bond fission channel at these shorter excitation wavelengths. Additional peaks in the H atom time-of-flight spectrum, most clearly evident following excitation at wavelengths in the range 213–220 nm, are interpretable in terms of secondary photolysis of the primary CH3S(X˜) fragments yielding thioformaldehyde (H2CS), primarily in its A˜ 1A2 excited electronic state. Symmetry arguments provide an explanation for this specific electronic branching in the near ultraviolet photolysis of CH3S fragments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468248
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  • 14
    Electronic Resource
    Electronic Resource
    Formation of striped surface phases by short-range forces (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8082-8086 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Anisotropic Cu–O islands form striped periodic supergratings upon annealing an oxygen-exposed Cu(110) surface. While the formation of these striped phases has been attributed to long-range repulsive forces between stripes, we show using the one-dimensional lattice gas model and Monte Carlo simulations that short range adsorbate–adsorbate interactions are capable of producing striped phases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468453
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  • 15
    Electronic Resource
    Electronic Resource
    The microwave spectrum, structure, and large amplitude motions of the methylacetylene⋅SO2 complex (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6512-6522 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotational spectra of five isotopomers of the methylacetylene⋅SO2 (MA⋅SO2) van der Waals complex have been observed with a Fourier transform microwave spectrometer. Each species showed two sets of rotational transitions, one associated with the A (m=0) and the other with the E (m=±1) methyl group internal rotation states. The rotational transitions of the isotopomers with S 16O2 and the doubly substituted S 18O2 also showed inversion splitting ranging from tens of kHz to a few MHz. This splitting was absent in the S 16O 18O isotopomers. The spectra of these species have been assigned and fit, yielding rotational constants, which allowed a complete determination of the structure of the complex. The SO2 was found to sit above the carbon–carbon triple bond, with one of the S–O bonds roughly parallel to the symmetry axis of methylacetylene. The centers-of-mass distance between the two monomers was determined to be 3.382(10) A(ring). The center frequencies of the inversion doublets (or quartets) were used in a fit of both the A and the E transitions; the barrier hindering the internal rotation of the methyl group was determined to be 62.8(5) cm−1. Based on the dependence of the inversion splitting on the transition dipole direction and isotopic substitution, the inversion motion was identified as an "in plane'' wagging of the SO2 relative to methylacetylene. A pure inversion splitting of 3.11 MHz (free from rotation) was extracted from the A-state spectrum of the normal species, from which an inversion barrier height of about 63 cm−1 was estimated.
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    URL: http://dx.doi.org/10.1063/1.468345
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  • 16
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    Electronically excited states in size selected solvated alkali metal atoms. I. First observation in NaNH3 by resonant two color two photon ionization (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6559-6564 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Resonant two color two photon ionization is used to investigate the first electronically excited state of NaNH3 as a first example of size selected, solvated alkali metal atoms systems (alkali⋅solventn). The observed spectrum shows a complex vibrational and rotational structure. The 0-0 transition is found at 12 220 cm−1 (1.52 eV), in good agreement with recent theoretical calculations. With increasing laser power ionic fragments of the molecule are seen, indicating possible new avenues to study the dynamics of charge transfer processes in this type of model system.
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    URL: http://dx.doi.org/10.1063/1.468349
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  • 17
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    Differences between L3 and L2 x-ray absorption spectra of transition metal compounds (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6570-6576 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The differences between L3 and L2 edges of 3d and 4d transition metal complexes and compounds in octahedral symmetry are discussed. The main origin of these differences are the multiplet effects due to the coupling of the 2p core wave function and the 3d and 4d valence wave functions. The 3d and 4d spin–orbit coupling is a second origin of difference. For 3d systems the multiplet effects dominate all other interactions and the L3 and L2 edge are completely mixed and reordered. For 4d systems the core hole spin–orbit coupling is large and the L3 and L2 are separated by about 100 eV with a ratio close to 2:1. The differences between the L3 and L2 edge originate from the weight transfer between the t2g and eg peaks due to the multiplet effect. This weight transfer is about 25% for the L3 edge and about 5% for the L2 edge, which implies that for a comparison to single-particle calculations the L2 edge is preferable to use. Partly filled 4d systems are low-spin and the occupation of the t2g states implies a decrease of the first peak. This decrease is stronger for the L2 edge, implying an increase in the L3:L2 ratio. For 4d5 systems transitions to the t2g hole are only possible at the L3 edge due to the combined effects of 4d spin–orbit coupling and the dd multiplet effects.
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    URL: http://dx.doi.org/10.1063/1.468351
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  • 18
    Electronic Resource
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    Calculation of internal conversion rate constants of single vibronic levels in S1 benzene (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6632-6639 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In the present work, a direct count procedure designed for the assessment of the decay rates of nonradiative processes in polyatomic molecules has been introduced. Using the described approach the internal conversion (IC) decay rates of a considerable number of single vibronic levels (SVL) in S1 benzene have been estimated. The decay rates of "bright'' SVLs were found to increase only slowly, while those of "dark,'' out-of-plane modes containing states grew far more rapidly with the excess vibrational energy Ev in S1. The observed behavior with Ev as well as the absolute magnitude of the calculated IC decay rates were in general conformity with the existing experimental evidence.
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    URL: http://dx.doi.org/10.1063/1.468457
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  • 19
    Electronic Resource
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    Measuring the structure of etched silicon surfaces with Raman spectroscopy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8055-8072 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the unenhanced, nonresonant surface Raman spectra of one monolayer of hydrogen bound to flat and stepped Si(111) surfaces prepared using a novel, aqueous fluorine etch. The orientation and normal mode composition of adsorbate vibrations are obtained from polarized, angle-resolved Raman spectra using a 3-layer dielectric model. This approach requires the experimental determination of both the anisotropy in the dynamic polarizability of the adsorbate bond and the effective dielectric constant in the vicinity of the adsorbate. The measured Si–H bond anisotropy is 0.263±0.028 in good agreement with gas phase measurements. The adsorbate dielectric constant is measured to be 3.78±0.20; this response is clearly nonlocal and predominantly due to polarization of the underlying silicon lattice. Using this technique, we find that the step dihydride on a Si[6(111)-(1¯1¯2)] surface is rotated 37°±4° from the surface normal in good agreement with the 31° predicted by ab initio cluster techniques, but significantly larger than the 12.5° predicted by pseudopotential slab calculations. In contrast to both theoretical predictions, the normal modes of this step dihydride display little concerted motion indicating that subsurface relaxation near the step edge reduces steric interactions much further than predicted. The observed anisotropic etch rates, evidenced by the production of atomically straight steps, are explained in terms of the measured distortion at the step edge.
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    URL: http://dx.doi.org/10.1063/1.468232
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    Stabilized hydrogen on Cs-modified Al(111): Evidence for formation of alkali aluminum dihydride surface complex (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8087-8093 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The adsorption of hydrogen on Cs-modified Al(111) surfaces has been studied using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). A stabilized hydrogen species which desorbs simultaneously with Cs at 480 K (γ hydrogen) appears in the presence of Cs and the ratio between the number of Cs and that of the γ hydrogen is 1:2, independent of Cs coverage, which is indicative of a substantial attractive interaction between Cs and the γ hydrogen. HREEL spectra of the γ hydrogen exhibit intense loss peaks at around 800 and 1700 cm−1. Detailed analyses of these loss peaks using isotope mixtures (H+D) indicate that the γ hydrogen is directly bound to Al with a geminal structure (AlH2). From the combined results of TPD and HREELS, it is considered that a cesium aluminum dihydride complex (CsAlH2) is formed on the surface, which is supported by ab initio calculations.
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    URL: http://dx.doi.org/10.1063/1.468235
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    Highly excited hydrogen molecules desorbed from a surface: Experimental results (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8152-8158 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rovibrational excitation of hydrogen molecules resulting from recombination of atoms on a metallic surface has been observed by means of an electronic collision method. We present the obtained H− experimental spectra reflecting the vibrational populations of the desorbed hydrogen molecules. This study brings to light the fundamental role of the nature of the metallic surface in the recombination process: excitation of higher vibrational levels (from v=5 to v=8) is observed only on those metal surfaces which strongly chemisorb atomic hydrogen. To explain these observations we propose the formation, on these surfaces, of a physisorbed H layer on the chemisorbed one.
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    URL: http://dx.doi.org/10.1063/1.468242
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    General equations for describing temperature-programmed surface segregation (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8200-8204 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Temperature-programmed segregation on the surface and diffusion from the surface into the bulk are shown to be described by an integral equation. This equation is integrated by taking into account that the diffusion coefficient rapidly increases with increasing temperature. The results obtained are used to study the effect of saturation and adsorbate–adsorbate interactions on the transient kinetics with two types of the initial conditions. In the first case, the surface and bulk are in equilibrium in the beginning, and then the surface coverage decreases with increasing temperature due to diffusion from the surface into the bulk. The second class of kinetics corresponds to the situation when in the beginning the surface coverage is much lower than the equilibrium one. In the latter case, the surface coverage first increases with increasing temperature due to segregation on the surface, then reaches maximum, and finally again becomes low.
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    URL: http://dx.doi.org/10.1063/1.468204
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    Laser-induced fluorescence detection of ClO radicals at 167–180 nm (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8262-8263 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: ClO radicals are detected by a laser-induced fluorescence technique, using the C 2Σ–X 2Π transition with a tunable vacuum ultraviolet laser system. Predissociation in the C 2Σ state is discussed in terms of the fluorescence lifetime and excitation spectra. The vibrational distribution of ClO produced by the reaction between O(1D) and HCl is measured.
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    URL: http://dx.doi.org/10.1063/1.468197
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    Study of the ArBr−, ArI−, and KrI− anions and the corresponding neutral van der Waals complexes by anion zero electron kinetic energy spectroscopy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6538-6551 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Three rare gas halide (RgX−) anions, ArBr−, ArI−, and KrI−, and the corresponding open-shell van der Waals complexes, ArBr, ArI, and KrI, were studied with anion zero electron kinetic energy (ZEKE) spectroscopy. Photodetachment of each anion accesses the three lowest-lying electronic states (the X1/2, I3/2, and II1/2 states) of the neutral complex. The spectra for each system reveal well-resolved progressions in the low frequency vibrations of the anion and the three neutral electronic states, providing a detailed spectroscopic probe of the Rg⋅X− and Rg⋅X interaction potentials. The line shapes observed in the ZEKE spectra are analyzed in terms of the line strengths of the underlying rotational transitions. From our data, we construct the potential energy curve for each neutral state as well as for the anion, and these interaction potentials are compared to potentials obtained from scattering and ion mobility experiments.
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    URL: http://dx.doi.org/10.1063/1.468500
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    Control of a quantum dynamics by a locally optimized laser field. II. Application to a system with dissipation (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6586-6592 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a theory for controlling the dynamics of a dissipative, quantum system with a laser field optimized locally in time. The theory is applicable to both weak and strong field control of the quantum dynamics. The theoretical groundwork is based on the equation of motion of the density matrix in Liouville space. Interactions between the molecules and the heat bath are taken into account within a Markov approximation. The derivation of the locally optimized laser field in a feedback form is based on the local optimization theory in the Hilbert space, proposed in a previous paper [M. Sugawara and Y. Fujimura, J. Chem. Phys. 100, 5646 (1994)]. The theory is applied to a simple, two-level quantum system with a dephasing constant. We present both the calculated time evolution of the off-diagonal density matrix element and that of the population of the states in the optimized laser field. These calculations show that the control of the system by the laser field is sufficient to avoid the dephasing effects. We discuss how the dephasing dynamics affects the optimization of the laser field.
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    URL: http://dx.doi.org/10.1063/1.468353
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    Dissociation dynamics of Ar+n (n=3–16) in collision with He and Ne (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6625-6631 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dissociation dynamics of Ar cluster ions, Ar+n (n=3–16), in collision with He and Ne atoms was investigated by measuring the absolute dissociation cross sections and the branching fractions of the product ions at the collision energies of 0.1–10 eV in the center-of-mass frame. The cross sections and the branching fractions were successfully explained in terms of collisional excitation of the parent cluster ion and subsequent dissociation of it; the collisional excitation was treated by a hard sphere collision model with the aid of the molecular dynamics (MD) and the diatomics-in-molecules (DIM) calculations reported previously, and the subsequent dissociation was explained by the Rice–Ramsperger–Kassel (RRK) theory.
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    URL: http://dx.doi.org/10.1063/1.468357
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    The thermal conductivity of argon in the critical region (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6944-6963 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the thermal conductivity of argon at temperatures from 302 K down to 150.8 K and at densities up to 25 mol L−1. The data were obtained with a steady-state method and we employed a guarded parallel-plate apparatus designed especially for investigating the thermal conductivity of fluids in the critical region. To interpret the data in the critical region a scaled crossover equation of state for argon in the critical region has been constructed. Equations for the thermal conductivity and viscosity of argon as a function of density and temperature are presented.
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    URL: http://dx.doi.org/10.1063/1.468322
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    Real time path integral methods for a system coupled to an anharmonic bath (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6708-6716 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We propose an efficient path integral scheme for calculating the quantum dynamics of an arbitrary one-dimensional system coupled nonlinearly to many anharmonic noninteracting "bath'' degrees of freedom. The starting point is an improved discretization of the path integral in terms of numerically constructed propagators [Chem. Phys. Lett. 193, 435 (1992)]. The resulting influence functional is comprised of one-dimensional correlation functions with step-structured time-dependent potentials and therefore is similar in structure to that employed in the spin-boson calculations of Coalson [J. Chem. Phys. 86, 995 (1987)]. In the present case, though, the influence functional is nonlinear and is computed using numerical iterative wave function propagation methods. Numerical tests on a system coupled to ten anharmonic oscillators demonstrate the efficiency of the proposed scheme, which requires numerical effort that scales only linearly with the number of anharmonic bath degrees of freedom.
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    URL: http://dx.doi.org/10.1063/1.468364
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    Auger electron–ion coincidence experiment on nitrogen molecule excited by electron impact (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6565-6569 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first Auger electron–ion coincidence experiment on the nitrogen molecule excited by electron impact is described. The kinetic energy releases of the N++ and the N+ fragments in the 43–72 eV binding energy range have been measured. The experimental results are compared with previous experimental data obtained by using synchrotron radiation and theoretical predictions.
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    Derivation and efficient implementation of the fast multipole method (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6593-6605 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The fast multipole method (FMM) of Greengard evaluates Coulomb interactions of point charges with computational requirements that increase linearly with the number of particles. In this work, the central transformations of the FMM are obtained in a very compact manner from simple algebraic manipulations of two addition theorems. The intermediate multipole and Taylor expansions are defined differently from previous work to yield simplified and more efficient transformations. Error estimates are obtained due to the effect of multipole truncation and the use of the multipole to Taylor transformation operator. Efficient implementation of the FMM for potential and forces is discussed, and calculations are presented that probe the accuracy and performance of the method.
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    Ring opening reaction dynamics in the reaction of hydrogen atoms with ethylene oxide (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6615-6624 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ethylene oxide, C2H4O, is a three-membered ring with a single oxygen atom bridging the two carbons. Reactions of H and D atoms with ethylene oxide have been studied in the gas phase to provide insight into the dynamics of three-membered ring opening. H atoms were produced by photolyzing HI in the wavelength range 240–266 nm. The channel leading to OH+C2H4 was monitored via laser-induced fluorescence (LIF) of the OH A 2Σ←X 2Π system. The D atom reaction yields OD with no hydrogen scrambling. With an available energy of 23 000 cm−1, the average OH D rotational energy is ∼350 cm−1 for OH(v=0) and OD(v=0) and ∼250 cm−1 for OD(v=1). OH(v=1) was not observed, while the OD(v=1) population was about one-tenth that of OD(v=0). There was no apparent bias in populations between Λ doublets in each of the spin–orbit states for both OH and OD. Doppler broadening of OH(v=0) rotational lines was measured to evaluate the average center-of-mass (c.m.) translational energy, which was found to be ∼2300 cm−1. On average, the ring opening process deposits ∼10% of the available energy into c.m. translation, ∼2% into OH rotation, and ∼88% into ethylene internal energy. Comparison with CH2CH2OH unimolecular dissociation dynamics and theoretical transition state calculations leads to a likely mechanism in which hydrogen abstracts oxygen via sequential C–O bond fission without involving a long-lived CH2CH2OH intermediate.
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    Cluster experiments in radio frequency Paul traps: Collisional relaxation and dissociation (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6666-6685 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents techniques to store, manipulate, and detect cluster ions in an rf Paul trap as an approach to study the physics and chemical physics of clusters. The trap has been designed to be the primary experimental environment in the sense that experimental manipulation and ion detection are performed in situ within the trap. Specific design considerations for the trap and rf electronics relevant to metal cluster experiments are discussed. We present the application of these techniques to measurements of trapped C+60 in order to estimate their limitations for studying metal cluster ions. This paper demonstrates the capability to nondestructively detect small variations in the number of trapped C+60 ions with an ion noise level of ≈100 ions limited by thermal current fluctuations. Trapped ion lifetimes of τion(approximately-greater-than)30 min are measured following the relaxation of C+60 translational energy to ≈300 K. Measurements of the collisional dissociation of trapped C+60 ions at rates 〈100 s−1 by He at ∼10−6 Torr have been performed and the products detected in situ by the mass distribution of the C+n products. Several possibilities for metal cluster experiments in rf Paul traps are discussed.
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    The role of adsorbate–adsorbate interactions in the rate oscillations in catalytic CO oxidation on Pd (110) (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6717-6727 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The CO+O2 reaction on Pd(110) exhibits kinetic oscillations above pO2 ≈ 10−3 Torr and bistability below this pressure. Based on the reversible formation of subsurface oxygen and the Langmuir Hinshelwood mechanism of catalytic CO oxidation, a mathematical model had been developed which described the occurrence of rate oscillations and most of the qualitative features of the oscillations. This model, however, failed to reproduce the change from bistability to oscillatory behaviour with increasing pO2. In this paper we demonstrate that by introducing repulsive interactions between COad and Oad, the subsurface oxygen model correctly reproduces the experimentally determined stability diagram in pO2,pCO parameter space. The effect of the repulsive interactions is to reduce the activation barrier for penetration of chemisorbed oxygen into the subsurface region, thus facilitating the formation of subsurface oxygen at high coverages. For the improved subsurface oxygen model a bifurcation analysis has been conducted in pO2,pCO parameter space. The influence of the constants in the model has been analyzed likewise with bifurcation theory.
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    Photodetachment in the gaseous, liquid, and solid states of matter (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6728-6742 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have made absolute cross section measurements of laser photodetachment of C6F−6 ions embedded in gaseous tetramethylsilane (TMS) and compared the results at low gas densities with measurements in nonpolar liquids and solids. The measurements indicate that the photodetachment cross section of C6F−6 in gaseous TMS is about three times larger than in liquid TMS. This is rationalized by considering the effect of the medium on both the photoabsorption and the autodetachment processes. The photodetachment cross section in both the gas and the liquid exhibits (at least) two maxima due to autodetaching negative ion states. It is argued that these are due to σ*→σ* transitions in C6F−6. The relative positions of these "superexcited'' anionic states did not change appreciably in going from the gas to the liquid and the solid, indicating similar influences of the medium on them. As expected, the photodetachment threshold in the condensed phase is shifted to higher energies compared to the gaseous phase. This shift is consistent with recent photoelectron studies of photodetachment of C6F−6 clusters. The present study clearly shows that the photodetachment from negative ions embedded in all states of matter proceeds directly or indirectly via negative ion autodetaching states, and that for nonpolar media, the effect of the medium can be accounted for by considering the macroscopic properties of the medium described by its dielectric constant ε and refractive index n.
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    Molecular dynamics simulation of ionic mobility. I. Alkali metal cations in water at 25 °C (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6964-6974 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe a series of molecular dynamics simulations performed on model cation-water systems at 25 °C representing the behavior of Li+, Na+, K+, Rb+, and Cs+ in an electric field of 1.0 V/nm and in its absence. The TIP4P model was used for water and TIPS potentials were adapted for the ion-water interactions. The structure of the surrounding water molecules around the cations was found to be independent of the applied electric field. Some of the dynamic properties, such as the velocity and force autocorrelation functions of the cations, are also field independent. However, the mean-square displacements of the cations, their average drift velocities, and the distances traveled by them are field dependent. The mobilities of the cations calculated directly from the drift velocity or the distance traveled by the ion are in good agreement with each other and they are in satisfactory agreement with the mobilities determined from the mean-square displacement and the velocity autocorrelation function in the absence of the field. They also show the same trends with ionic radii that are observed experimentally; the magnitudes are, however, smaller than the experimental values in real water by almost a factor of 2. It is found that the water molecules in the first solvation shell around the small Li+ ion are stuck to the ion and move with it as an entity for about 190 ps, while the water molecules around the Na+ ion remain for 35 ps, and those around the large cations stay for 8–11 ps before significant exchange with the surroundings occurs. The picture emerging from this analysis is that of a solvated cation whose mobility is determined by its size as well as the static and dynamic properties of its solvation sheath and the surrounding water. The classical solventberg model describes the mobility of Li+ ions in water adequately but not those of the other ions.
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    Molecular dynamics simulations of a flexible molecule in a liquid crystalline solvent (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7027-7036 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations of a hexane molecule in isotropic, nematic, and smectic phases are reported. The interactions between the solvent molecules is modeled using the Gay–Berne potential and the hexane–solvent potential is modeled as an explicit site–site interaction. The conformational distributions are reported of the hexane molecule at a fixed temperature but at different densities in isotropic, nematic, and smectic phases, and these are compared with the results from Monte Carlo simulations on an isolated molecule at the same temperature. The positions and orientations of the hexane molecule at each time step in the molecular dynamics simulations are used to calculate interproton dipolar coupling constants. These data are used to test the mean field models which have been applied with dipolar couplings obtained previously from nuclear magnetic resonance (NMR) experiments to yield conformational distributions.
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    Stratification-induced order–disorder phase transitions in molecularly thin confined films (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6865-6873 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By means of grand canonical ensemble Monte Carlo simulations of a monatomic film confined between unstructured (i.e., molecularly smooth) rigidly fixed solid surfaces (i.e., walls), we investigate the mechanism of molecular stratification, i.e., the tendency of atoms to arrange themselves in layers parallel with the walls. Stratification is accompanied by a heretofore unnoticed order–disorder phase transition manifested as a maximum in density fluctuations at the transition point. The transition involves phases with different transverse packing characteristics, although the number of layers accommodated between the walls remains unchanged during the transition, which occurs periodically as the film thickens. However, with increasing thickness, an increasingly smaller proportion of the film is structurally affected by the transition. Thus, the associated maximum in density fluctuations diminishes rapidly with film thickness.
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    Aqueous solvation dynamics with a quantum mechanical Solute: Computer simulation studies of the photoexcited hydrated electron (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6902-6916 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have used molecular dynamics simulation to explore aqueous solvation dynamics with a realistic quantum mechanical solute, the hydrated electron. The simulations take full account of the quantum charge distribution of the solute coupled to the dielectric and mechanical response of the solvent, providing a molecular-level description of the response of the quantum eigenstates following photoexcitation. The solvent response function is found to be characterized by a 25 fs Gaussian inertial component (40%) and a 250 fs exponential decay (60%). Despite the high sensitivity of the electronic eigenstates to solvent fluctuations and the enormous fractional Stokes' shift following photoexcitation, the solvent response is found to fall within the linear regime. The relaxation of the quantum energy gap due to solvation is shown to play a direct role in the nonradiative decay dynamics of the excited state electron, as well as in the differing relaxation physics observed between electron photoinjection and transient hole-burning (photoexcitation) experiments. A microscopic examination of the solvation response finds that low frequency translational motions of the solvent play an important role in both the inertial and diffusive portions of the relaxation. Much of the local change in solvation structure is associated with a significant change in size and shape of the electron upon excitation. These results are compared in detail both to previous studies of aqueous solvation dynamics and to ultrafast transient spectroscopic work on the hydrated electron.
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    The direct correlation functions and bridge functions for hard spheres near a large hard sphere (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6975-6978 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The recent Monte Carlo data of Degrève and Henderson for the density profiles of hard spheres near a large hard sphere are used to obtain direct correlation functions and bridge functions for this system both directly, using the Ornstein–Zernike relation and an approximation, due to Verlet, for the bridge function.
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    Density-functional modeling of structure and forces in thin micellar liquid films (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6979-6991 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent equilibrium force measurements on aqueous films of surfactant above the critical micelle concentration show oscillations for film thicknesses up to 50 nm. To model this phenomenon we express the micellar contribution to the disjoining pressure in terms of thickness-dependent inhomogeneous micelle number density distributions through the film. Density functional theory is used to calculate micelle density profiles, presuming the micelles to behave as charged spheres interacting with each other, and with the film interfaces, through screened-Coulomb potentials. The background electrolyte permits dilute micellar solutions to act as concentrated systems exhibiting pronounced layering in the film. For a 0.1 M sodium dodecylsulfate (SDS) film we find up to five micellar layers for a film thickness equal to ten micelle diameters (d), the layer separation scaling with the effective diameter (deff/d=1.86) which includes the micelle Debye atmosphere. The peaks are largest near the interfaces and decay toward the bulk density at the film midplane. The corresponding disjoining pressures show oscillations with the same distance scaling between the branches as in the density profiles; these values are consistent with experiment. With decreasing film thickness, the (meta-)stable disjoining pressure regions represent micellar layers in the film being forced closer together, raising the pressure until the interior layer is expelled, allowing more space between the remaining micellar layers at that thickness.Repulsive (positive) disjoining pressures result from layer separations less than the corresponding bulk value whereas attractive (negative) regions represent more distance between layers than that in the bulk. The 0.2 M SDS disjoining pressure isotherm exhibits one additional layer than the 0.1 M case for thicknesses up to 50 nm. The pressure magnitudes of the former case are about twice that of the latter. Addition of ionic salts greatly inhibits the long-range micellar structuring. For SDS foam films, predicted disjoining pressures are much higher than measured values. Comparison with cetyltrimethyl-ammonium bromide (CTAB) micellar films in the surface forces apparatus, however, shows near quantitative agreement. The nature of the confining interfaces thus plays a key role in supporting the internal micellar structuring.
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    Quantum molecular dynamics simulations of liquid alkalies (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7048-7057 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We employ a unified molecular dynamics (MD), quantum mechanical approach to simulate the behavior of large collections of atoms at finite temperature. The nuclei are moved according to classical mechanics while the forces are computed via quantum mechanical models. Two approaches have been used: (1) the extended Hückel method, which is an approximate molecular orbital approach and (2) density functional theory based on the local density approximation and plane wave pseudopotential formulation. We compute properties from the MD trajectories of up to 4 ps duration for samples containing up to 250 atoms of lithium, sodium, and potassium. These results are compared to both previous calculations and experimental results.
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    Relative role of surface interactions and topological effects in nuclear magnetic resonance of confined liquids (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7074-7081 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The relative roles of surface and topological effects on the nuclear relaxation rates T−11, T−12, and T−11ρ of polar or nonpolar liquids in porous sol-gel silica glasses are identified via their very different pore size and frequency dependences. On the basis of theory, experimental relaxation rates, and molecular dynamics simulations for the modeled porous systems, the 1/Ti's are interpreted in terms of a linear combination of bulk, confinement, and surface effects: 1/Ti = 1/Tibulk + ai/R2+ bi/R, where R is the average pore size and ai and bi are given in terms of the usual relaxation parameters of the studied molecular species. This simple expression which allows the determination of the relative roles of surface and topological effects has been used to fit the observed 1H NMR relaxation rates as a function of pore size and frequency for methylcyclohexane, nitrobenzene, pyridine, and toluene both for nonmodified and surface modified porous silica glasses. Using this method, the surface (∝1/R) and pure geometrical (∝1/R2) relaxation contributions are evaluated and the surface and translational correlation times are calculated. More generally, the experimental data allow us to explain the following seemingly paradoxical results obtained for confined liquids: (i) The pure confinement effect is independent of the polarities of the liquid molecules in pores and is very sensitive to the frequency. (ii) The finding of the frequency variation of T−11 and T−11ρ both for polar or nonpolar liquids confined to small pores, shows that the geometrical confinement effects dominate over the surface interaction effects at low frequency and for small pores.
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    Electric field modulation studies of optical absorption in CdSe nanocrystals: Dipolar character of the excited state (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7122-7138 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Stark effect on the electronic absorption spectrum of CdSe nanocrystals has been studied for nanocrystals ranging in size from 80 to 20 A(ring) in diameter. For all but the smallest clusters, a second derivative line shape is observed, indicative of a dipole moment in the excited state. This result is independent of the surface modification and appears in both CdS and CdSe systems. The Δμ ranges from 15±10 D in the smallest clusters and up to 100±10 D in the largest; however, the increase is not monotonic, and in the very largest clusters studied (d(approximately-greater-than)70 A(ring)), the dipole moment decreases. The dipolar character is lost in clusters less than 25 A(ring). These results can be explained by a model in which there is resonance of an interior state with a surface state at a particular size, with the mixing occurring on a preferred axis.
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    Reversible charge transfer in dielectric restricted and fractal solids: Low concentration of acceptors (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7169-7173 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the influence of dielectric properties of medium with dielectric constant ε in the charge transfer processes in the restricted and fractal solid spaces. It is shown that this influence is determined by the space restriction and its regularity. The increasing of ε promotes charge separation in restricted and fractal systems. The fractal properties grow weakly with increasing of ε; its increasing transfers the irregular fractal space into the regular one with respect to the charge transfer at ε(very-much-greater-than)α. The corresponding dependencies of the charge transfer efficiency on the value of ε are presented. The space restriction and breach of its homogeneous with alteration of ε give qualitatively identical results. The obtained results are good for low concentration of acceptors.
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    The percolation-to-cluster transition during spinodal decomposition of an off-critical polymer mixture. Observation by light scattering and optical microscopy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7181-7184 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Direct observation of the percolation-to-cluster transition of an off-critical mixture of polybutadiene and polyisoprene was achieved by optical microscopy. According to optical evidence, the transition occurs in a slow and a fast step not revealed in parallel light scattering experiments.
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    Reassignment of the vibrational structure of the Huggins absorption band of ozone (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5431-5437 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new assignment of the vibrational structure of the Huggins band of ozone is given. The assignment is consistent with the symmetry requirements on changes in the asymmetric stretching (v3) quantum number and with data on the differences in the location of vibronic bands between 16O3 and 18O3. Based on the reassignment of the spectrum, new values for the vibrational constants for 16O3 and 18O3 in the excited electronic state responsible for the Huggins system are calculated. The reassignment of the Huggins system provides a simple explanation for several otherwise puzzling observations concerning the spectrum, and suggests that the Huggins and Hartley band systems arise from transitions to different electronic states, in accord with previous molecular orbital calculations.
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    Extreme-ultraviolet photodissociation of N2O in superexcited states (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5473-5483 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We demonstrate the observation of neutral dissociation, which provides the possibility of a spectroscopy of highly lying superexcited states. The yield spectrum of the undispersed fluorescence radiation of wavelength λf in the region 113≤λf≤180 nm from excited neutral fragments in the photodissociation of N2O is presented in the region of excitation photon wavelength λex in the region 30≤λex≤111 nm (photon energy region 41.3–11.2 eV). We show the evidence of selective or preferential neutral dissociation in the decay of superexcited N2O in competition with autoionization; in particular, the evidence for an important role of neutral dissociation of superexcited N2O followed by the production of ionic fragments. We also show the neutral dissociation of superexcited states with the character of double-holed doubly excited states located for 30≤λex≤60 nm. The aspect of superexcitation (bound channel of electron promotion) in the region of inner-valence excitation is discussed in relation with free (continuum) channels.
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    Infrared absorption of cis–cis peroxynitrous acid (HOONO) in solid argon (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5494-5499 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Nitric acid (HONO2) isolated in solid argon at 12 K was irradiated with 193 nm emission from an ArF excimer laser. Recombination of the photofragments led to formation of peroxynitrous acid (HOONO) in various conformers. In addition to previously reported lines due to trans–perp HOONO, lines at 3285.4, 1600.3, 1395.0, 927.2, 794.3, and 629.1 cm−1 were observed; they are assigned to cis–cis HOONO having a five-membered hydrogen-bonded ring. The observed vibrational frequencies and the corresponding isotopic shifts of both conformers are in agreement with recent theoretical calculations. Cis–cis HOONO was photolyzed much more rapidly than trans–perp HOONO upon irradiation at 308 nm with a XeCl laser. Lines due to a HO2...NO complex were also observed; they disappeared upon irradiation with the globar source of the infrared spectrometer. The mechanism of formation of various conformers of HOONO is discussed.
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    Bound-state wave functions from coupled channel calculations using log-derivative propagators: Application to spectroscopic intensities in Ar–HF (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5578-5584 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method for obtaining wave functions from coupled-channel bound state calculations using log-derivative propagators is presented. Bound states occur at energies for which the inward and outward log-derivative solutions match at a central point in the propagation; at such energies, the log-derivative matching matrix has one eigenvalue which is zero. The wave function at the matching point is the eigenvector corresponding to this zero eigenvalue. The wave function at other points can be obtained by back-substitution in the log-derivative propagation equations. The method is tested by calculating infrared intensities for the Ar–HF van der Waals complex, using the H6(4,3,2) potential.
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    Ab initio calculations on monohalogenophosphanes PH2X (X=F,Cl,Br,I), and experimental detection and characterization of PH2F and PH2Cl by high resolution infrared spectroscopy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5585-5595 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The harmonic and anharmonic force fields of the title compounds have been calculated at the ab initio self-consistent-field level using effective core potentials and polarized double-zeta basis sets. Additional calculations for PH2F employ larger basis sets and include electron correlation. Many rovibrational constants are predicted theoretically. The infrared spectra generated from the ab initio data have guided the experimental identification of PH2F and PH2Cl in the gas phase. High resolution Fourier transform infrared spectra of these unstable molecules have been recorded for the first time. Rotational analyses for several bands are reported which provide accurate ground state constants and a precise characterization of a number of vibrationally excited states. The accuracy of the ab initio predictions for PH2F and PH2Cl is evaluated by comparisons with these experimental data.
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    Collisional relaxation of translational energy and fine-structure levels of the O(3Pj) atom created in the photodissociation of SO2 at 193 nm (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5647-5651 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Collision-induced energy transfer and intramultiplet relaxation processes of O(2p4 3Pj) j=0, 1, and 2 have been studied, using a vacuum-ultraviolet laser-induced fluorescence technique. The O(3Pj) atoms are produced from the photodissociation of SO2 at 193 nm. Doppler profiles of the O(3P2) atoms have been measured as a function of time delay between pump and probe laser pulses. The translational energy and j-level population of the O(3Pj) atoms are thermalized at 300 K after a number of collisions with either He, Kr, Xe, N2, or CO2. A Monte Carlo calculation using the hard-sphere collision model provides the cross sections for relaxation of the translational energy; 21±3 A(ring)2 for He, 25±3 A(ring)2 for Kr, and 25±5 A(ring)2 for Xe. Assuming the principle of detailed balancing among the j levels, the cross sections for the collision-induced intramultiplet transition over the collision energy range 1–6 kcal mol−1 are obtained from the time evolution of the j-level populations: cross sections for the j=2→1 transition are 2.4±1.2 A(ring)2 for Kr and 1.4±0.7 A(ring)2 for Xe.
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    Velocity dependence of free ion production in K(np)–C2Cl4, CS2, and O2 collisions: Internal-to-translational energy transfer (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5672-5676 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rates for free ion production through electron transfer in collisions between K(np) Rydberg atoms and C2Cl4, CS2, and O2 are measured as a function of Rydberg atom velocity for intermediate values of n, 13≤n≤21. The data show that postattachment interactions between the product positive and negative ions are important and can lead to conversion of internal energy from the negative ion, which is formed in an excited state, into translational energy of the product ion pair. This energy conversion occurs without ion–ion neutralization and can stabilize the negative ion against dissociation or autodetachment.
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    Time-resolved multiphoton ionization study of the 102 nm state of NO (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5708-5716 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have used subpicosecond deep UV laser pulses (near 205 nm) in a time-resolved (2+1) multiphoton ionization study of the two-photon state of NO at 102 nm via a one-photon resonance with the A 2Σ+(3sσ) (ν=2) state. The two-photon excited state lifetime exhibits a strong pressure dependence from which the collisionless lifetime and the bimolecular depopulation or quenching rate constant were obtained. The large quenching rate constant strongly suggests that the state is a medium (10–12) n Rydberg level in the series converging to the ground electronic state of NO+. The subnanosecond collisionless lifetime implies that this state is subject to predissociation and/or autoionization.
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    Solvation of electronically excited I2− (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5731-5755 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction potentials between the six lowest electronic states of I−2 and an arbitrary discrete charge distribution are calculated approximately using a one-electron model. The model potentials are much easier to calculate than ab initio potentials, with the cost of a single energy point scaling linearly with the number of solvent molecules, enabling relatively large systems to be studied. Application of the model to simulation of electronically excited I−2 in liquids and CO2 clusters is discussed. In a preliminary application, solvent effects are approximated by a uniform electric field. If electronically excited (2Πg,1/2) I−2 undergoes dissociation in the presence of a strong electric field, the negative charge localizes so as to minimize the total potential energy. However, in a weak field the negative charge localizes in the opposite direction, maximizing the potential energy. Based on a study of the field-dependent potential surfaces, a solvent-transfer mechanism is proposed for the electronic relaxation of 2Πg,1/2I−2, in contrast to the conventional view of relaxation via electron transfer.
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    Rotational energy transfer in vibrationally excited acetylene X˜ 1Σg(ν2‘=1,J‘):ΔJ propensities (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5804-5817 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A complete set of state-to-state rotational energy transfer rate constants has been measured for acetylene–acetylene collisions at room temperature under single collision conditions. Initial rotational states (Ji=5,7,...,25) were prepared and final states (Jf=1,...,25) interrogated. The measurements were carried out in a typical gas phase pump and probe arrangement. The initial vibrationally excited state was prepared by stimulated Raman pumping using strong Q-branch transitions. State preparation via this branch produces an isotropic spatial distribution of the excited state which is important for data analysis. Narrow bandwidth lasers ensure single rotational state selectivity. The rotational distribution after collisions is monitored by time-delayed laser-induced fluorescence via the A˜ 1Au(ν3' = 1)←X˜ 1Σg(ν2‘ = 1) transition. In general, the rate constants decrease exponentially with the transferred rotational energy. The complete rate constant matrix can consistently be described by a simple parameter set within the dynamical infinite order sudden power approximation. In addition to this general behavior a significant ΔJ propensity of the rate constants is observed. Using the energy corrected sudden approximation with a power law basis an excellent match, reproducing the ΔJ propensities, to the rate constant matrix is obtained, again with a single set of parameters.
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    Orthogonal polynomials in phase space and classical analogs to quantum eigenstates with an application to the transmission flux for the potential step (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5847-5852 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We introduce a set of orthogonal polynomials in phase space with similar properties, and which can be reduced to the usual Hermite polynomials. Based on these polynomials, we develop a method that allows the finding of classical analogs to quantum eigenstates. This method is applied to the free particle scattered by a potential step and the transmission flux is calculated and compared with the quantum one.
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    Adsorption energies of NH3 and NH+4 in zeolites corrected for the long-range electrostatic potential of the crystal (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5865-5874 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The calculation of adsorption energies in zeolites in the cluster approximation has the advantage of a reduced computational effort compared to that of the periodic approach. However, in the cluster approximation, the long-range electrostatic effects of the infinite crystal are ignored and there are boundary effects. In order to remove a part of the disadvantages of the cluster approach, we developed a method to embed a cluster in a zeolite crystal by imposing an electrostatic potential on it. This potential adds the long-range electrostatic effects of the crystal and subtracts the electrostatic potential of the boundary of the cluster. It is calculated from the charge distribution of the crystal obtained with an ab initio calculation using the crystal program. We calculated the adsorption energies of NH3 and NH+4 on three different clusters embedded in the potential of a chabazite crystal, and we have compared them with the adsorption energies of NH3 and NH+4 in the crystal. If a cluster is used that has dangling bonds four, or more, bonds away from the adsorbate, i.e., the adsorption site is described well covalently, embedding the cluster reduces the error in adsorption energies (relative to the crystal calculations) from ≈20 to ≈2–3 kJ/mol.
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    Femtosecond studies of interparticle electron transfer in a coupled CdS–TiO2 colloidal system (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6222-6225 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the direct femtosecond measurements of the electron transfer process from CdS to TiO2 particles in a coupled colloidal system. The electrons initially created in the conduction band and quickly trapped at the liquid–solid interface in aqueous CdS colloids are found to be transferred to the colloidal TiO2 particles with a time constant of 2 ps, resulting in significantly slower electron–hole recombination. These coupled semiconductor colloids provide a potentially useful system for achieving efficient charge separation, which is important for many chemical reactions involving heterogeneous electron transfer.
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    Pulsed gradient spin echo nuclear magnetic resonance for molecules diffusing between partially reflecting rectangular barriers (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4599-4609 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The application of pulsed gradient spin echo nuclear magnetic resonance (NMR) to the case of molecules trapped between two plane parallel boundaries, has been examined theoretically, with computer simulation and by experiment. A new closed-form analytic expression is obtained for the averaged propagator and the echo attenuation when the walls have finite relaxivity and this expression is verified by computer simulations. It is shown that "diffraction'' effects are still strongly apparent when wall relaxation is taken into account and that deviations in the barrier spacing parameter obtained from the position of the echo minimum, are weak. In particular we show that for the pulsed gradient spin echo (PGSE) pulse separation time on the order of a2/2D, the deviation is less than 10% provided that the relaxation is not so severe as to reduce the zero gradient signal amplitude below 10% of its unrelaxed value. We further examine the influence of finite gradient pulse and find, as with wall relaxation, that diffusion during the gradient pulse has the effect of shifting to higher q the position of the first minimum in the echo diffraction pattern. The diffusive diffraction effect is demonstrated experimentally using a stack of pentane-filled microcapillaries of wall spacing 100 μm, and fits to the data yield realistic values for the known experimental parameters.
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    Line shapes and forced autoionization of the 8s and 9s, v=1 Rydberg states of NO (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4617-4620 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report line shapes for several rotational levels of the 9s, v=1 state of NO, lying just above the autoionization threshold, and the 8s state, just below threshold. The 8s levels exhibit unexpectedly large Stark shifts, giving way at higher fields to multiline Stark spectra, and eventually to Lorentzian autoionizing resonances. The large low-field shifts are tentatively attributed to an unusual "field-induced deperturbation'' process. At zero field, the 8s states have lifetimes of at least 20 ns, indicating that predissociation does not play a major role for these states.
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    Comparison of infrared absorption intensities of benzene in the liquid and gas phases (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4610-4616 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents a comparison of the absolute infrared absorption intensities in the liquid and gas phases for the four infrared active fundamentals of benzene. In Herzberg's notation these are ν12 (∼3070 cm−1), and ν4 (∼675 cm−1). Published data are used, including the recently published spectra of liquid benzene that have been accepted by the International Union of Pure and Applied Chemistry as secondary intensity standards. The present results agree qualitatively with the conclusions drawn in 1970 that the intensity Aj of ν12 is much smaller for the liquid than for the gas, and those of ν13, ν14, and ν4 are all larger for the liquid. The inclusion of measurements made since 1970 should make the quantitative results reported here the most reliable. However, two quite different values have been reported in the 1980's for the intensity of ν14 in the gas phase, and both are considered. The comparison for ν14 is also complicated by the existence of weak bands in the spectrum of the liquid that are not observed in that of the gas. It is noted in this work that the traditional comparison, of the areas under the molar absorption coefficient spectra, Aj, for the gas and liquid through the Polo–Wilson equation, has the drawback that the ratio expected if the dipole moment derivative is unchanged is different for each band as well as for each liquid.A much more convenient ratio, that equals unity for all bands of all liquids under the traditional assumptions, is proposed through the imaginary molar polarizability spectrum of the liquid. The magnitudes of the transition moments and the dipole moment derivatives with respect to the normal coordinates under the double harmonic approximation are calculated from the measured intensities for the gas and liquid phases. It is found that the dipole moment derivative of ν12 is 24% smaller in the liquid than in the gas and that of ν13 is 18% larger. The dipole moment derivative of ν4 is unchanged by condensation. The change in the dipole moment derivative of ν14 is not clear, because of the uncertainty in the gas phase intensity and because of the uncertain origin of the intensity of the additional bands in the liquid.
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    URL: http://dx.doi.org/10.1063/1.467448
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    A site selective luminescence excitation and Raman scattering study of Eu(CF3SO3)3-PPO; ion association and ionic exchange rate (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4621-4627 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The luminescence of Eu3+ in a salt–polymer complex, poly(propylene oxide) containing Eu(CF3SO3)3 salt, was studied using broad band and site-selective excitation. Two distinctly different types of local structure around Eu3+ were detected. To investigate and compare the local structure around the SO3CF−3 anions with that around the Eu3+ cations, a Raman scattering study of the SO3 symmetric stretch vibration was performed in the same system. The vibrational data clearly reveal that also the anions are subject to two types of local environment. It is found that neither of the two states can be attributed to "free'' (completely solvated) ions, which implies the existence of two different anion–cation configurations. Temperature and concentration dependent studies of the site-selective Eu3+ luminescence and the SO3 vibrational spectrum show that the relative amount of ions in the two states is, within the experimental accuracy, constant. However at elevated temperatures there is a rapid exchange between the two states not found at lower temperatures. The characteristic time scale for the ionic exchange is found to be closely related to the structural relaxation time of the polymer segmental mobility. The findings support the idea that ionic diffusion in salt–polymer complexes is assisted by the local polymer chain mobility which is used in models to explain polymer electrolyte performance.
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    URL: http://dx.doi.org/10.1063/1.467450
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    Symplectic integrators for large scale molecular dynamics simulations: A comparison of several explicit methods (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4062-4072 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We test the suitability of a variety of explicit symplectic integrators for molecular dynamics calculations on Hamiltonian systems. These integrators are extremely simple algorithms with low memory requirements, and appear to be well suited for large scale simulations. We first apply all the methods to a simple test case using the ideas of Berendsen and van Gunsteren. We then use the integrators to generate long time trajectories of a 1000 unit polyethylene chain. Calculations are also performed with two popular but nonsymplectic integrators. The most efficient integrators of the set investigated are deduced. We also discuss certain variations on the basic symplectic integration technique.
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    URL: http://dx.doi.org/10.1063/1.467523
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    The chemical potential in atomically inhomogeneous fluids in external force fields by computer simulation (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4098-4102 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The accuracy and practicality of the Widom fictitious-particle insertion method for determining the chemical potential is tested for very inhomogeneous fluids in strong external potentials by a series of molecular dynamics computer simulations. The chemical potential determined in this way is found, as predicted, on an atomic scale to be independent of position in the fluid to a few percent even when the density varies by up to a factor 80. This severe test means that this method of determining chemical potential is established as reliable and accurate in any well-found computer simulation even when the system is very inhomogeneous.
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    URL: http://dx.doi.org/10.1063/1.467527
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    Water above its boiling point: Study of the temperature and density dependence of the partial pair correlation functions. I. Neutron diffraction experiment (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4123-4132 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Neutron diffraction data on water, employing the technique of hydrogen/deuterium isotope substitution, are reported at three thermodynamic states above the boiling point. The structural information is analyzed in terms of the partial radial distribution functions, OO, OH, and HH, which are extracted from the neutron data. It is found that temperature affects mainly the medium and longer range order in the liquid, while density plays a significant role in controlling the degree of hydrogen bonding. To understand the structure of water obtained from these data it appears that many-body cooperative interactions have to be correctly accounted for.
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    URL: http://dx.doi.org/10.1063/1.467462
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    Electronic properties of a dilute polarizable fluid: A Green's function approach (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4133-4142 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a new theoretical analysis of a harmonic model of a polarizable fluid. In this Drude oscillator model, particles with classical mechanical translational degrees of freedom contain internal quantum harmonic oscillators, representing electronic degrees of freedom, which are coupled by dipolar interactions. We relate the calculation of optical properties such as the density of states and the absorption spectrum at finite wave vector to the determination of a Green's function. A graphical analysis of this Green's function is shown to suggest a hierarchy of approximations. Calculations based on two members of this hierarchy are compared to recently published simulations of optical properties for this model. Because of the long range of dipolar interactions, this model shows interesting optical properties even in the limit of low particle number density. Our method is shown to agree well with simulation data in this limit.
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    Computer simulation of vapor–liquid equilibria of linear quadrupolar fluids. Departures from the principle of corresponding states (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4166-4176 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vapor–liquid equilibria of different quadrupolar linear Kihara fluids have been studied, by using the Gibbs ensemble Monte Carlo technique. Coexistence curves for fluids with elongations L*=L/σ=0.3, 0.6, and 0.8 and different quadrupoles are given. We analyze the effect of quadrupole moment on critical properties. Quadrupole moment increases the critical temperature, pressure, and density. The magnitude of the increase depends on both anisotropy and quadrupole moment. A new way of reducing the quadrupole is proposed, so that the variation of critical properties due to the quadrupole follows a universal behavior. Quadrupole provokes deviations from the principle of corresponding states. A broadening of the coexistence curve is observed due to the quadrupole. The quadrupole moment increases the slope of the vapor pressure curve vs temperature inverse. Simulation data are used to describe vapor–liquid equilibria of carbon dioxide. Good agreement between simulation and experiment is achieved.
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    URL: http://dx.doi.org/10.1063/1.467467
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    Off-specular reflection from flat interfaces (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4198-4204 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reflection of x ray or neutron from a flat interface with density or compositional fluctuations was studied. An integral equation relating the reflection intensities to the interfacial fluctuations was derived in the Fraunhofer limit. At the specular condition this integral equation, although derived for the off-specular intensities, has an expression identical to a well-established one for the specular component. This integral equation was applied to a flat surface onto which gold spheres of uniform size were randomly adhered. The out-of-plane components for both the real wave and the time reversed wave were approximated by the solution from the specular component. Based on the calculated off-specular intensities near the specular region, an approximation scheme was proposed to estimate the in-plane correlation length.
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    URL: http://dx.doi.org/10.1063/1.468464
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    Two-dimensional model of geometric effects in thin film crystal orientation (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4229-4235 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple two-dimensional model is employed to derive the fraction of oriented crystalline area in thin film crystallization processes, and to assess the influence of various physical parameters upon the extent of orientation. The final expressions, which are given as averages over probability distributions, are shown to be precise via comparisons with numerical calculations. We find that for a given film thickness the seed (nucleus) density and nucleus orientation probability distribution have a dramatic influence upon the extent of orientation, while the crystal growth rate anisotropy has a smaller effect.
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    URL: http://dx.doi.org/10.1063/1.467473
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    Polymer chains with excluded volume: Critical exponents from free energy optimization (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4268-4276 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A self-consistent approach to the polymer excluded-volume problem is proposed, based on the Domb–Gillis–Wilmers (DGW) probability distribution for all atom pairs W(rj)∝ρθjexp(−Dρδj), ρj=rj/〈r2j〉1/2, rj being the interatomic distance between atoms separated by j bonds. As usual, we take 〈r2j〉∝j2ν for j(very-much-greater-than)1. The DGW distribution represents the simplest realistic assumption beyond the Gaussian distribution and is well supported by both theoretical and numerical evidence. Upon optimization of the chain free energy with respect to the mean-square Fourier amplitudes, the numerical values θ=0.31 and ν=0.604 are obtained, in rough agreement with the expected results θ(approximately-equal-to)0.7, ν=0.588. The results are shown to be invariant with the type of the polynomial expansion yielding the potential of mean force; the numerical disagreement may be due to the assumption of a single, average value of θ for all atomic pairs, consistent with a configurationally uniform chain model. Unlike with the Gaussian approximation, no factors comprising ln(j), ln[ln(j)]... are obtained within the expression of 〈r2j〉. Constraining to the values required by the DGW distribution the quadratic and quartic averages of the Fourier components constructed with the chain bond vectors, we obtain an approximate distribution function with a quartic polynomial of the chain coordinates within the exponent; this polynomial is proportional to the potential of mean force, useful in chain dynamics.
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    Symmetry-adapted perturbation theory potential for the HeK+ molecular ion and transport coefficients of potassium ions in helium (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4697-4707 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction potential for the HeK+ system has been computed as the sum of attractive and repulsive contributions due to the electrostatic, exchange, induction, and dispersion interactions using the symmetry-adapted perturbation theory and a high-level treatment of electron correlation. The zero of the theoretical potential occurs at 4.704 bohr and the minimum occurs at 5.418 bohr where the potential value is −0.779 mhartree. The potential supports 36 bound rovibrational levels, and the ground state of the HeK+ molecular ion is bound by 125.1 cm−1. For all interatomic distances the ab initio potential agrees very well with the empirical potential obtained by direct inversion of the K+ mobilities in gaseous helium and disagrees with the potential obtained from the ion–beam scattering cross section data. The ab initio potential has been used to compute the transport coefficients of potassium ions in helium gas over a wide range of temperature and reduced field strength. A very good agreement of the calculated transport coefficients with the preponderance of the existing experimental data is observed. This agreement leads us to believe that the present interaction potential is accurate to within a few percent at all separations between 4.0 and 10.0 bohr and, consequently, represents the most accurate potential available for the HeK+ system.
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    URL: http://dx.doi.org/10.1063/1.467459
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    Experimental and theoretical study of the O+HCl transition state region by photodetachment of OHCl− (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4708-4721 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present measured and calculated photodetachment spectra of OHCl−, and we interpret the results in terms of the vibrational structure of the transition state of the O+HCl→OH+Cl reaction. The measured spectra exhibit two distinct features—an intense broad peak at high electron kinetic energies and a less intense shoulder at lower energies. Superimposed on these broad features are several sharper structures, but they are barely discernible from noise in the spectrum. To interpret these spectra, we have used a recently developed global 3A‘ potential surface for the O+HCl reaction to calculate Franck–Condon factors, using an L2 method (i.e., expansion in terms of square integrable basis functions) to approximate the scattering wave functions on the reactive surface. Assignment of the spectrum has been assisted using the results of quantum coupled channel calculations for the same surface. The resulting calculated spectrum shows the same broad features as the measured spectrum. There is also fine structure with spacings and energies that are similar to the experiment, but specific features do not match. To interpret both the broad and fine features in the theoretical spectrum, a hierarchical analysis is applied wherein this spectrum is decomposed by a tree construction into components of increasingly higher resolution. The physical meaning of each of these components is then determined by plotting "smoothed states'' that are obtained from the tree coefficients. This leads to the conclusion that the two broad features in the spectrum are made up of progressions in hindered rotor states of the Cl–OH complex, with the most intense feature corresponding to OH(v=0) and the weaker shoulder corresponding to OH(v=1). There is evidence for Feshbach resonance features in the v=1 feature, but it appears that most of the fine structure is due to hindered rotor states.
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    Chain desorption from a semidilute polymer brush: A Monte Carlo simulation (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4379-4390 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We discuss the dynamic properties of a semidilute grafted polymer layer exposed to pure solvent. When the grafting energy of the head groups of the chains is finite, the chains desorb and are expelled from the layer. We combine Monte Carlo simulations using the bond fluctuation model to self-consistent mean field calculations and a scaling analysis. Chain desorption can be seen as a two step process. For strongly grafted polymers the limiting step is the desorption of the head group. The chain is then expelled by the osmotic pressure gradient. A chain cut off the wall is expelled at a constant velocity of its center of mass. The velocity decreases as the inverse of the molecular weight and increases with the grafting density. In the early stages of the expulsion the tension of the monomers close to the wall relaxes and the chain retracts. The retraction is independent of the molecular weight. Our most important result is that the desorption of the head group is a local process with a characteristic time independent of molecular weight. The desorption time increases exponentially with the grafting energy, it decreases as a power law of the grafting density. The exponent is close to 2 but the precise value is difficult to extract from the simulation.
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    Coupling between diffusion and deformation of gels in binary solvents: A model study (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4407-4417 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study theoretically the dynamics of chemically crosslinked gels in binary solvents on the basis of the Flory-type model and nonequilibrium thermodynamics. We find with the aid of numerical integration of the evolution equation that an instability occurs in the gel upon changing the composition of the surrounding solution. This instability is caused by a marked difference of timescales between quick mutual diffusion of solvent molecules and slow deformation of the gel network. Profiles of solvent composition and network deformation are also described in some details.
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    URL: http://dx.doi.org/10.1063/1.467490
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    Computational studies of halogen chemistry on rare-gas surfaces. III. Photodissociation in submonolayer chlorine films on Ar(111) and Xe(111) (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4433-4444 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation reaction of submonolayer molecular chlorine films on rare-gas surfaces has been studied using molecular dynamics computer simulation. In particular, we have considered the coverage and temperature dependence of escaping photofragment yield, angular distribution, and translational energy distribution of the reaction h(ν=29 661 cm−1) +Cl2(1Σ)−Cl2[aitch-theta]ads−Rg(111)[T]→2Cl⋅(1Π)[E=9699 cm−1]+Cl2[aitch-theta]ads−Rg(111)[T'], where E is the initial potential energy of the dissociating fragments, aitch-theta indicates the film coverage, T is the substrate temperature, and Rg indicates the rare-gas substrate which is either argon or xenon. Even at the submonolayer coverages studied here (X2[adsorbed]/Rg[surface]〈1), fewer than half of the photodissociation fragments escape the surface. The mean translational energy of the escaping fragments as a function of coverage indicates that, on average, escaping fragments suffer collisions and lose energy before leaving the surface. At all coverages, the translational energy distribution is roughly Gaussian and peaked at approximately one-third of the total photodissociation energy. However, at higher coverages, the escaping fragment may actually carry more than half of the photodissociation translational energy. The average direction of escaping photofragment velocities as a function of coverage generally reflects the original diatom orientation in the adlayer but is shifted toward the surface normal. With increasing coverage, the angular distribution of fragment velocities goes from roughly Gaussian in the range between 0° and 90° to highly peaked about the surface normal.
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    Computational studies of halogen chemistry on rare-gas surfaces. II. Structure of chlorine and bromine submonolayer films on Ar(111) and Xe(111) prepared by molecular beam dosing (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4418-4432 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The deposition by molecular beam dosing of halogen molecules on rare-gas surfaces has been studied with molecular dynamics simulation. Specifically we have considered films formed by the sequential adsorption reactions: X2g+X2[aitch-theta]ads−Rg(111)[T]→X2(ads)−X2[aitch-theta]ads −Rg(111)[T], where aitch-theta is the film coverage defined by X2[adsorbed]/Rg[surface], T indicates the substrate temperature, X2 is the halogen adsorbate which is either chlorine or bromine, and Rg indicates the rare-gas substrate which is either argon or xenon. The structure of halogen adlayers was studied as a function of coverage for films grown on rare-gas substrates at different temperatures. Chlorine and bromine films on argon exhibit orientational ordering and islanding with increasing coverage. The tendency of the halogen diatoms to align along the surface normal with increasing coverage is strongly enhanced by higher temperatures in the case of chlorine on xenon, moderately enhanced in the case of chlorine on argon, and unchanged in the case of bromine on argon. Chlorine and bromine films form three-dimensional aggregated structures on argon at 15 and 25 K. Chlorine forms a two-dimensional amorphous layer on xenon at 15 K and a highly ordered layer at 50 K. The dynamics of the molecular adsorption event were studied at three different coverages: aitch-theta=0.05, 0.25, and 0.5. At all these coverages, we observed finite possibility of diffusional motion of adsorbate on the surface immediately after it lands: This was not seen at zero coverage.
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    Optical absorptions of Li atoms in mixed Ar/Xe matrices (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2678-2683 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the results of matrix isolation spectroscopy experiments on mixed Ar/Xe matrices containing Li atoms produced by laser ablation of solid lithium. Optical absorption spectra of Li/Ar/Xe matrices containing ∼0.01% guest Li atoms and ≈0, 3%, 10%, 30%, 50%, 70%, 90%, 97%, and 100% Xe as the matrix host are included. In all cases well defined "triplet'' absorption features (i.e., three main peaks) are observed for the Li atom 2p←2s absorption. We also present new data on the photobleaching of the well known "red triplet'' absorption in Li/Xe matrices, which show changes to the fine structure observed on the sharp 655 nm component. In these dilute guest systems, the Li atom absorption line shape is determined completely by guest–host interactions, which depend strongly on the local Li atom trapping site structure. In single Rg host matrices, it is possible that the trapping site structures may correspond to single or multiple Rg atom substitutional sites in otherwise crystalline regions of the rare gas solid. In these cases, the observed triplet line shapes would be due to the removal of the threefold degeneracy of the excited Li atom 2p state by dynamical distortions of the system away from the high symmetry equilibrium trapping site structures. In the mixed Ar/Xe matrices, the Li atom trapping sites necessarily have lower equilibrium or static symmetries due to the possibly amorphous nature of these solids, and to the differences in the Li–Ar and Li–Xe interactions. The observed spectra in mixed host matrices thus contain contributions from the many and varied Li atom trapping site structures, yet they still exhibit the familiar triplet absorption pattern. While we do not settle the long-standing question as to the crystalline vs amorphous nature of the single Rg host matrices, the present observations do provide new data for the comparison of the relative importance of static vs dynamic effects on the spectra of matrix isolated alkali atoms.
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    URL: http://dx.doi.org/10.1063/1.467648
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    Simplification of the transition state concept in reactive island theory: Application to the HCN(arrow-right-and-left)CNH isomerization (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4790-4802 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactive island (RI) theory of chemical reaction rates is modified so that a precise knowledge of the transition state is not required. This revised form of the theory is applied to a realistic two (J=0) and three degree of freedom model (J≠0) of the isomerization HCN(arrow-right-and-left)CNH. The detailed reaction dynamics is examined and understood in terms of the cylindrical manifolds embedded in the molecular phase space. Of special interest we find a T-shaped conformer in addition to the HCN and CNH moieties at J=0. Rates of unimolecular decay are examined by numerical simulation and theory. Good agreement is found between theory and simulation except for a case where a substantial amount of phase space is consumed by regular motion. In all cases studied, the theory is a significant improvement over the Rice–Ramsperger–Kassel–Marcus (RRKM) theory, which is up to three orders of magnitude too large.
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    Nascent internal state distributions of ZnH(X 2Σ+) produced in the reactions of Zn(4 1P1) with some alkane hydrocarbons (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4803-4808 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactions of Zn(4 1P1) with CH4, C2H6, C3H8, and C(CH3)4 were studied by employing a laser pump-and-probe technique. The nascent rotational and vibrational state distributions of ZnH(X 2Σ+) were determined. These distributions were compared with those predicted by statistical models. The distributions for C(CH3)4 resembled to the statistical ones, while those for simple alkanes such as CH4 were a little hotter than the statistical ones. These results suggest that the reaction proceeds via a relatively long-lived insertive complex. There was no great difference in the production yields of ZnH, although that for CH4 was the largest.
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    Millimeter-wave rotational spectrum of H2SiO (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2729-2733 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first high resolution rotational spectrum of H2SiO in the gas phase has been obtained. This molecule was identified among several short-lived unknown species produced by a low power "abnormal'' glow discharge in a mixture of SiH4, O2, and Ar. On/off modulation of the discharge allowed lines belonging to each species to be characterized by their lifetime measured using a digitizing oscilloscope. In total 107 lines of H2SiO were measured between 140 and 470 GHz. Evidence for a possible large amplitude motion has been found from anomalies in the fit of close lying Ka doublets.
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    URL: http://dx.doi.org/10.1063/1.467654
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    Dissociation dynamics of CH+4 core ion in the 2A1 state (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2720-2728 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Threshold-photoelectron photoion coincidence (TPEPICO) spectra of CH4 have been observed with synchrotron radiation at the excitation to the 2A1 (ν1=0−3) ionic states as well as to the 4pt2 Rydberg (ν1=0−4) states. In all the TPEPICO spectra observed, the CH+3 band shape was almost rectangular, which suggests that the translational and internal energy distributions of CH+3 are very narrow. The total kinetic energy releases (KERs) have been estimated from the CH+3 band shape. As a result, it was found that the CH+3 species were in an electronically excited state. There was a narrow distribution of the total KERs and similarity in the TPEPICO CH+3 band shapes between the spectra at the 2A1 ionic state and the 4pt2 Rydberg state excitations, which led to the conclusion that the Rydberg electron is just a spectator and the dissociation of the core ion plays an important role in dissociation through the 4pt2 Rydberg state. Similar results have also been obtained for CH+2 and CH+ productions. However, on the other hand, an H+ fragment has been observed only at the 2A1 state excitation. It showed a band with a long tail in the slower flight time region. The total average KERs and the decay rates have been estimated from band shape simulation. From these results, it has been found that a dissociation limit of the H+ ion exists just below the 2A1 ionic state. The dissociation mechanisms through the 4pt2 Rydberg state have been discussed in detail in comparison with those of the 2A1 ionic state.
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    URL: http://dx.doi.org/10.1063/1.467653
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    Valence bond calculations of the potential energy surface for CH4→CH3+H (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4826-4831 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A valence bond study of the potential energy surface for methane CH4→CH3+H is performed at the 6–31G level using (i) a valence bond self-constituent field (VB-SCF) method; (ii) a valence bond configuration interaction (VBCI) method; and (iii) an antisymmetrized product of strong-orthogonal geminals (APSG) method (also in VB form). The calculations show that, although the energies are somewhat inferior (on an absolute scale) to those obtained in very large CI calculations, the VB reduced potential energy surfaces behave better, in the intermediate range 2–3 A(ring), than those obtained using (i) the Møller–Plesset fourth-order perturbation (MP4) approximation, (ii) configuration interaction with all singles and doubles (CISD), and (iii) coupled clusters with all singles and doubles (CCSD). The results are in very good agreement with those obtained from multi-reference configuration interaction (MR-CISD) calculations. The lower absolute energies obtained in the very extensive CI calculations indicate a better description of electron correlation, both in the molecule and in its dissociation products, but evidently they do not imply a better overall description of the PE surface. The remarkable fact is that a single VB structure, with carefully optimized orbitals, provides an excellent description of the whole dissociation process.
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    URL: http://dx.doi.org/10.1063/1.467405
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    Excited potential energy surfaces of CH3SH from the ab initio effective valence shell Hamiltonian method (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4832-4841 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and first and second 1A‘ potential surfaces of methyl mercaptan (CH3SH) are calculated as a function of the C–S and S–H bond coordinates using the ab initio effective valence shell Hamiltonian (Hν) method. The computations for this highly nontrivial system provide the first serious tests for choosing restricted valence spaces and for computing global potential energy surfaces with the Hν methods. The quasidegeneracy constraints on the Hν method suggest choosing a valence space which consists of the two (3a‘ and 10a') highest energy occupied orbitals in the ground state and the three lowest a' unoccupied orbitals. The global potential surfaces are computed with a modest basis, but larger basis set computations at selected geometries test convergence for vertical excitation energies, ionization potentials, and C–S and S–H bond energies. The calculations are compared to both experiment and other calculations for this system. The computations assist in the interpretation of CH3SH photodissociation dynamics observed by Butler and co-workers.
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    URL: http://dx.doi.org/10.1063/1.467406
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    Chemical binding and electron correlation effects in x-ray and high energy electron scattering (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4842-4854 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The total, inelastic and elastic intensities for x-ray and high energy electron scattering from the ten-electron molecules CH4, NH3, H2O, and HF have been calculated with configuration interaction wave functions. The probability density of the interelectronic distance or the radial intracule density is extracted from the intensities. The importance of basis sets, chemical binding, and electron correlation to the scattered intensities and the radial intracule density has been examined. It has been found that scattered intensities predicted with correlated wave functions agree well with experiment. Based on the electron pair distribution, the exchange and correlation holes in molecular systems have been briefly discussed.
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    URL: http://dx.doi.org/10.1063/1.467407
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  • 85
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    Photodecomposition of Mo(CO)6/Si(111) 7×7: CO state-resolved evidence for excited state relaxation and quenching (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2929-2939 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: State-resolved detection techniques have been used to characterize the ultraviolet photodecomposition dynamics of Mo(CO)6 on Si(111) 7×7 at 100 K. Details of the excitation/fragmentation mechanism including adsorbate energy transfer were examined by measuring the cross sections and the internal and translational energies of the photoejected CO from submonolayer through multilayer coverage regimes. The CO energy distributions are found to be independent of Mo(CO)6 coverage, and can be characterized by two components with markedly different mean energies. In contrast to the coverage independence of the measured energy disposal, the cross section was found to decrease by a factor of 3 from multilayer coverages to submonolayer coverages.
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    URL: http://dx.doi.org/10.1063/1.467605
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    Analysis of two ion swarms in dynamic equilibrium: Simultaneous determination of two drift velocities and two reaction frequencies (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2949-2952 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a drift-tube study, four simple formulas, based on a dynamic equilibrium analysis of two gaseous ion swarms, are proposed to determine simultaneously the forward and reverse reaction frequencies together with two drift velocities. The formulas are put to the test for (N+4,N+2)/N2 system with two arrival time spectra (ATS) generated from the experimental data previously reported at E/N=150 Td (the ratio of electric field strength to gas number density: 1 Td=10−17 V cm2). The percent differences between the data and the determined values are estimated to be 0.8%, −1.4%, 6.8%, and 4.9%, for the drift velocities of N+4 and N+2 ions and the forward and reverse reaction frequencies, respectively. For the same system, two experimental ATS obtained at 180 Td by McKnight et al., being supposedly in near equilibrium, are directly examined by this method.
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    URL: http://dx.doi.org/10.1063/1.467607
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    Some predictions relevant to future spectroscopic observation of S1 vinylidene (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3001-3005 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equation-of-motion coupled cluster method is used to obtain a precise estimate of the lowest adiabatic singlet–singlet excitation energy (S1←S0) of vinylidene. As photodetachment spectroscopy offers a promising means for producing the first excited singlet state, harmonic vibrational frequencies of S1 and the extent of mixing between the corresponding normal modes and those of the anion are also calculated. To calibrate the calculations and provide a basis for empirical correction, parallel studies of the previously observed T1 and T2 triplet states are reported. The adiabatic excitation energy [S1←S0] is estimated to be 3.12±0.05 eV, suggesting a photodetachment threshold energy of 3.61±0.05 eV. Progressions in the totally symmetric C–C stretch and H–C–H bending modes should be pronounced in the spectrum, with the extent of mode mixing similar to that found for the S0←anion and T1←anion processes, but differing in phase.
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    URL: http://dx.doi.org/10.1063/1.467613
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    The chemical bonds in CuH, Cu2, NiH, and Ni2 studied with multiconfigurational second order perturbation theory (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4893-4902 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The performance of multiconfigurational second order perturbation theory has been analyzed for the description of the bonding in CuH, Cu2, NiH, and Ni2. Large basis sets based on atomic natural orbitals (ANOS) were employed. The effects of enlarging the active space and including the core-valence correlation contributions have also been analyzed. Spectroscopic constants have been computed for the corresponding ground state. The Ni2 molecule has been found to have a 0+g ground state with a computed dissociation energy of 2.10 eV, exp. 2.09 eV, and a bond distance of 2.23 A(ring). The dipole moments of NiH and CuH are computed to be 2.34 (exp. 2.4±0.1) and 2.66 D, respectively.
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    URL: http://dx.doi.org/10.1063/1.467411
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    The geometric phase in two electronic level systems (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4903-4913 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effects of the geometric phase on vibronic states associated with the lower potential surface of two electronic level Hamiltonians are examined. We obtain the general formula for the gauge potential arising from the vibronic interaction. It is shown that this gauge potential is split into a topological part and a magnetic part, where the topological part gives rise to the phase factor of +1 or −1 when it is integrated along a closed trajectory in the nuclear coordinate space, and the magnetic part gives rise to a contribution depending on the local character of the trajectory and exists only in systems without time-reversal symmetry. For particular examples, we consider the E⊗e and E⊗(b1+b2) Jahn–Teller systems with strong vibronic interactions. It is demonstrated that the ground states have vibronic standing wave states whose nuclear probability density distributions are localized in one of the equivalent minima on the lower potential surface. We also consider Zeeman splittings of degenerate vibronic states, where the degeneracy arises from time-reversal symmetry.
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    URL: http://dx.doi.org/10.1063/1.467412
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  • 90
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    The thionitroxyl free radical (H2NS) and its ionic counterparts (H2NS+ and H2NS−): A theoretical and experimental study (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4885-4892 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An experimental and theoretical characterization has been carried out for the thionitroxyl radical (H2NS) and its ionic counterparts (H2NS)+ and (H2NS)−. Experimentally, (H2NS)+ and (H2NS) have been detected by a combination of collisional activation and neutralization-reionization mass spectrometry. Thiourea has been used as a precursor. Theoretically, the neutral and ionic potential energy surfaces have been explored using (PU)MP4/6-311++G(3df,2p) energies based on (U)MP2/6-311++(d,p) geometries. Predictions for some spectroscopic and thermochemical properties have been made on the basis of molecular orbital calculations and empirical corrections. The rearrangements leading to the (HNSH) and (NSH2) isomeric species and the loss of hydrogen or other fragmentations have also been examined. The (HNSH) and (NSH2) species are less stable than their respective thionitroxyl isomers in all states considered. For the (H2NS)+ cation, an H loss constitutes the lowest energy fragmentation followed by an H2 elimination giving NS+ and a cleavage of the N–S bond. The thermochemical data are predicted as follows: ΔH0f,0 (H2NS)=31.2±3 kcal/mol, IEa(H2NS)=88.±0.2 eV, EAa(H2NS)=1.4±0.2 eV, PA(H2NS−)=357.5±3 kcal/mol as compared with the experimental value of 356.8±3 kcal/mol, ΔH0f,0(HNS)=52.6±3 kcal/mol and ΔH0f,0(H2NSH)=13.4±3 kcal/mol.
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    URL: http://dx.doi.org/10.1063/1.467410
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    Nonlinear molecular properties using biorthogonal response approach (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4914-4919 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we report the use of extended coupled cluster functional of Arponen, Bishop, and co-workers to implement a stationary biorthogonal response approach. The objective of this is to calculate nonlinear molecular properties like hyperpolarizability, etc. in a more convenient way.
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    URL: http://dx.doi.org/10.1063/1.467413
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    Equations of state valid continuously from zero to extreme pressures for H2O and CO2 (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3111-3116 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new form of equation of state is proposed for use over extremely wide ranges of pressure where conventional equations fail. In particular, fluids including H2O and CO2 as well as argon, etc., remain more compressible at very high densities than can be represented by typical equations with van der Waals or Carnahan and Starling repulsive terms. The new equation is fitted to the data for H2O and CO2 over the entire range from the vapor and liquid below the critical temperature to at least 2000 K and from zero pressure to more than 105 bar (10 GPa) with good agreement. The extension of the equation for mixed fluids is discussed.
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    URL: http://dx.doi.org/10.1063/1.467624
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    A Monte Carlo method for simulating associating fluids (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3147-3156 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present association biased Monte Carlo (ABMC), a Monte Carlo method, which is ergodic, microscopically reversible, and specifically designed to simulate associating fluids with long-ranged center-to-center interactions. The canonical ensemble (NVT) algorithm biases sampling to regions of configuration space where particle association is likely to occur, and provides efficient simulation of associating fluids over a broad range of densities. The usual canonical ensemble (NVT) thermodynamic variables (ensemble average internal energy and pressure), as well as the pair distribution functions are presented. The distributions of associated clusters are presented at a selection of state points and are compared with predictions of thermodynamic perturbation theory for the model system. We also present the simulation results for a symmetric, binary associating fluid with a single site on each particle.
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    URL: http://dx.doi.org/10.1063/1.467562
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    Computer simulation studies of a model system for liquid crystals consisting of semiflexible molecules (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3157-3171 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple steric model of so-called rfr (rigid–flexible–rigid) molecules is developed. It consists of two rigid spherocylindrical fragments linked by a flexible alkyl chain. Molecular dynamics simulations of 128 model molecules are carried out in a periodic box, and the influence of density and alkyl chain length on the molecular order and the self-diffusion is studied. Different liquid crystalline phases are found which are stable within the time scale of the simulations. In agreement with experimental results the nematic order parameters and the stability of the phase, as indicated by the location of the isotropic/liquid crystalline phase transition, show an odd–even effect with respect to the number of methylene groups in the alkyl chain. A separation of rigid and flexible fragments is observed leading to smectic liquid crystalline phases.
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    URL: http://dx.doi.org/10.1063/1.467563
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    Multistate quantum Fokker–Planck approach to nonadiabatic wave packet dynamics in pump–probe spectroscopy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3049-3061 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The quantum Fokker–Planck equation of Caldeira and Leggett is generalized to a multistate system with anharmonic potentials and a coordinate dependent nonadiabatic coupling. A rigorous procedure for calculating the dynamics of nonadiabatic transitions in condensed phases and their monitoring by femtosecond pump–probe spectroscopy is developed using this equation. Model calculations for a harmonic system with various nonadiabatic coupling strengths and damping rates are presented. Nuclear wave packets in phase space related to electronic coherence are shown to provide an insight into the mechanism of nonadiabatic transitions. The Green's function expression for these wave packets is used to explore possible algorithms for incorporating electronic dephasing in molecular dynamics simulations of curve crossing processes.
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    URL: http://dx.doi.org/10.1063/1.467618
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    The inclusion of connected triple excitations in the equation-of-motion coupled-cluster method (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3073-3078 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the implementation of connected triple excitations in the equation-of-motion (EOM) coupled-cluster (CC) method for excitation energies for the first time. The reference state is described by the complete CC singles, doubles, and triples (CCSDT) method. Excited states are generated from the reference state wave function by the action of a linear excitation operator including single, double, and triple excitations. The excited state wave functions and energies are obtained by diagonalizing the effective Hamiltonian e−THeT, where T is the cluster operator for the reference state, in the space of singly, doubly, and triply excited determinants. Comparison is made with full configuration interaction excitation energies for several examples (CH+, Be, SiH2, and CH2). These show that EOM-CCSDT is able to describe states which are doubly excited relative to the reference state, as well as singly excited states. Calculations of several excitation energies of BH using an extended basis set are also reported, and show good agreement with experiment.
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    URL: http://dx.doi.org/10.1063/1.467620
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    Polymers in quenched random environment: Weak replica symmetry breaking (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3104-3110 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The case of polymers in strong quenched disorder is investigated. The disorder is modeled by a quenched random potential which is sampled by a flexible polymer chain. It is shown that the problem, originally formulated in terms of Edwards type path integrals, can be transformed into an effective field theory. Localized phases of the polymer, where the size of the chain is independent of its contour length, i.e. R2∝(1/Δ)2/(4−d), can be found by a weak breaking of the replica symmetry. Δ is a measure for the disorder. The same formalism predicts at d=4 an essential singularity R2 ∝exp(2π2/Δ) for vanishing disorder Δ→0.
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    URL: http://dx.doi.org/10.1063/1.467623
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    Turing patterns and solitary structures under global control (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3135-3146 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Striped Turing patterns and solitary band and disk structures are constructed using a three-variable multiscale model with cubic nonlinearity and global control. The existence and stability conditions of regular structures are analyzed using the equation of motion of curved boundaries between alternative states of the short-range component. The combined picture of transitions between striped and spotted patterns with changing level of global control is in qualitative agreement with the results of the computer experiment by Middya and Luss [J. Chem. Phys. 100, 6386 (1994)].
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    Structure and energetics of van der Waals complexes of carbon monoxide with rare gases. He–CO and Ar–CO (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4964-4974 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy surfaces for Ar–CO and He–CO were calculated at the fourth order Møller–Plesset perturbation theory and analyzed using perturbation theory of intermolecular forces. Both the complexes reveal only one minimum related to the approximately T-shaped geometry. For Ar–CO, our best ab initio estimates of Re and De are 3.70 A(ring) and 496 μhartrees, respectively, and the optimal angle Rg–com–O is about 80°. For He–CO, our best Re and De are 3.4 A(ring) and 100 μhartrees, respectively, at the optimal angle Rg–com–O of 70°. Our geometrical parameters agree very well with the experimental data. Our ab initio well depths are estimated to be within ±5% in error and are expected to be the most accurate in the literature so far. The De values were obtained with extended basis sets which included bond functions. Basis set effects on the dispersion and electrostatic correlation terms that are caused by bond functions were also analyzed. Both complexes are bound by dispersion forces, but the anisotropy of the interaction is determined by the exchange repulsion component. This anisotropy may be interpreted in terms of the relative concentrations and depletions in the diffuse region of the CO charge density. The electrostatic and induction effects proved to be negligible.
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    URL: http://dx.doi.org/10.1063/1.467419
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    On the internal temperature in polymer glass simulations (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3326-3333 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An approximate theory, including effects of the density, for the equilibrium properties of a polymer lattice model (bond-fluctuation model) is presented and compared with Monte Carlo simulations. Effects of the density are taken into account by balancing the volume requirements of the individual bond vectors with the pressure that a melt exerts at a given density. This simple theory yields rather accurate (to within a few percent) estimates for the chain length and temperature dependence of various quantities that scan different length scales of the polymers (bond length, radius of gyration, etc.). With the inclusion of the density, one can also design an improved definition of an internal temperature of the melt, which is distinct from the external temperature of the surrounding heat bath. When the melt falls out of equilibrium during the vitrification process, this internal temperature is expected to measure the temperature that the melt actually experiences, and may thus serve to eliminate the nonequilibrium effects from the simulation data. The extent to which this expectation is borne out by a comparison with the simulation is addressed.
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    URL: http://dx.doi.org/10.1063/1.467580
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