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  • Articles  (21,647)
  • Springer  (18,851)
  • American Institute of Physics (AIP)  (2,040)
  • Blackwell Publishing Ltd  (756)
  • 1990-1994  (21,647)
  • 1950-1954
  • 1990  (21,647)
  • Chemistry and Pharmacology  (12,734)
  • Medicine  (7,542)
  • Energy, Environment Protection, Nuclear Power Engineering  (4,567)
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  • Articles  (21,647)
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  • 1990-1994  (21,647)
  • 1950-1954
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  • 1
    Electronic Resource
    Electronic Resource
    Rotational energy transfer in OH (A 2Σ+, v'=0): A method for the direct determination of state-to-state transfer coefficients (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6453-6462 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have determined state-to-state rate coefficients for rotational and fine structure transitions of OH (A 2Σ+, v'=0) in thermal collisions with He and Ar at 300 K. The temporal evolution of single fluorescence lines within the A–X, 0–0 band of OH were measured, exciting either the F2(4) or F2(5) state by a nanosecond laser pulse. The OH radical was produced in a discharge flow cell, containing predominantly He or Ar, at various pressures between 1 and 6 mbar. The time resolution in the experimental setup was sufficient to evaluate the rotational energy transfer coefficients directly from the time dependence of two fluorescence lines. The observed average rate coefficients for collisions of OH (A, v'=0) with Ar are approximately 3 times larger than those with He. The two rare gases show different qualitative behavior. Whereas the almost isoenergetic transitions with ΔJ=1 and ΔN=0 are favored in collisions with Ar, those with ΔJ=ΔN=−2 are favored in collisions with He. In addition, a strong preference for transitions conserving the parity of the OH, a propensity rule, previously reported for rotational relaxation in the A state of OH, was found for collisions with He but not for collisions with Ar. Our experimental results for He and for Ar are in good agreement with recent quantum mechanical calculations of the energy transfer coefficients.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458962
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  • 2
    Electronic Resource
    Electronic Resource
    Validity of time-dependent self-consistent-field (TDSCF) approximations for unimolecular dynamics: A test for photodissociation of the Xe–HI cluster (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6484-6490 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation dynamics of a collinear model of the van der Waals cluster Xe–HI is used as a testing ground for time-dependent self-consistent field (TDSCF) approximations. In this study, the quantum-mechanical TDSCF and a combined classical/quantal TDSCF (in which the light atom is treated quantum mechanically, the heavy atoms are treated classically) are compared to numerically exact wave packet calculations. Very good agreement is found between the TDSCF approximations and the exact result over the entire subpicosecond time duration of the process. In particular, all the properties related to the quantal degree of freedom in the combined quantal/classical TDSCF method reproduce almost perfectly the exact results. However, the classical mode in the hybrid approximation is somewhat less well described due to insufficient representation of energy transfer between the modes. The conclusions are very promising as to the applicability of TDSCF methods, in particular the hybrid quantal/classical scheme to more complex systems in which only a few degrees of freedom can be treated quantum mechanically.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458965
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  • 3
    Electronic Resource
    Electronic Resource
    Theoretical study of the dissociative recombination of NO+ with slow electrons (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6491-6501 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The multichannel quantum defect theory (MQDT) has been applied to the dissociative recombination of NO+ by collisions with slow electrons. The Rydberg states (superexcited states of the second kind) are fully incorporated by using the quantum defect functions. Five valence-type dissociative superexcited states of the first kind (A' 2Σ+, I 2Σ+, B 2Π, L 2Π, B' 2Δ) are all considered as the dissociation channels. Since the recently obtained quite reliable information on the superexcited states is employed in the calculation, the present results are considered to be the most accurate ones at this moment. Energy dependence of the cross sections shows intriguing resonance structures and varies from state to state, reflecting the relative dominance of the three basic important quantities: quantum defect function, electronic coupling, and Franck–Condon factor. The results are compared with other calculations and the available experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458966
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  • 4
    Electronic Resource
    Electronic Resource
    Rotational heating and cooling during the evaporation of atoms from excited clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6502-6507 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model based on phase space theory has been used to examine the effect that multiple fragmentations from excited, weakly bound cluster ions have on the final rotational temperatures of the reaction products. It is predicted that small clusters, such as Ar+2 and Ar+3, could emerge from a multiple reaction sequence with comparatively high rotational temperatures. In contrast, large clusters, e.g., Ar+40, experience slight rotational cooling when they undergo (approximately-equal-to)15 fragmentation steps. In each case, these results can be rationalized in terms of the relative magnitudes of the rotational and orbital angular momenta at each reaction step. Recent experimental observations on cluster ions are discussed in the light of predictions from the model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458967
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  • 5
    Electronic Resource
    Electronic Resource
    Primary events following electron injection into water and adsorbed water layers (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6535-6542 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The initial stages of the evolution of an electron injected into bulk water (at 300 K) and into thin water films (1–4 monolayers) adsorbed on a Pt(111) substrate at 50 K are investigated. It is shown that for electrons injected into bulk water with an initial translational kinetic energy between 1.54 and 6.18 eV (i.e., subexcitation energies), the electron momentum time-correlation function 〈pˆ(0)pˆ(t)〉, decays to zero on a time scale of less than 1 fs, reflecting strong backscattering of the electron by the water molecules. On this time scale the electron propagation in the medium is dominated by elastic processes. Furthermore, during this initial stage the system is well represented by a static aqueous medium. Transmission of electrons injected into thin films of adsorbed water is also dominated by elastic scattering. The dependence of the electron transmission probability on the film thickness and the initial injection energy are in accord with recent experimental results of photoinjected electrons into adsorbed water films.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458969
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  • 6
    Electronic Resource
    Electronic Resource
    Diode laser probing of I*(2P1/2) Doppler profiles: Time evolution of a fast, anisotropic velocity distribution in a thermal bath (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6543-6553 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The relaxation of a nonthermal translational population distribution of fast I*(2P1/2) atoms dilutely dispersed in a gaseous bath at thermal equilibrium is studied by time-resolved Doppler spectroscopy. The fast, anisotropic velocity distribution of I* atoms is produced by pulsed laser photolysis of n-perfluoropropyl iodide (n-C3F7I) at 266 nm. A frequency-narrowed, GaAsInP diode laser is tuned across the iodine (2P1/2,F=3←2P3/2,F=4) transition at 1315 nm to measure the Doppler gain profile of the I* photofragments. The velocity distribution is expressed as a separable product of a radial speed function and an angular function describing the anisotropy. The collision-induced time evolution of both the speed and anisotropy components of the nascent velocity population distribution relaxing to form a 300 K Maxwellian equilibrium distribution is determined. The thermalization dynamics of I* are studied for a heavy bath gas (n-C3F7I) and a light (He) bath gas. In the case of the heavy bath gas the anisotropy is removed by collisions 2.5 times faster than the speed is thermalized, while for the light bath gas the anisotropy and speed relaxation occur on the same time scale. The velocity and angular distributions of the I* photofragment from the 266 nm photolysis of n-C3F7I are also reported.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458970
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  • 7
    Electronic Resource
    Electronic Resource
    Reaction path power series analysis of NH3 inversion (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6570-6577 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new tool for reaction-path analysis of chemical dynamics is introduced, namely, series expansion of reaction-path potentials. This analysis involves the expansion of vibrational frequencies in powers of a reaction coordinate measuring progress along a suitably defined reaction path. The method is applied to generalized-normal-mode vibrational frequencies along two choices of reaction path for the inversion of ammonia. The leading nonzero terms in the power series are calculated by Møller–Plesset second-order perturbation theory and are fully converged with respect to basis set and the step size used for calculating the minimum energy path and the Hessian.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459697
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  • 8
    Electronic Resource
    Electronic Resource
    Photodissociation of acetylene: Determination of D00 (HCC–H) by photofragment imaging (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6578-6584 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Acetylene cooled in a He supersonic expansion is photodissociated by excitation in the 201–216 nm region of the A˜ 1Au −X˜ 1∑+g transition. Subsequent ionization of the H-atom fragments by 2+1 (243 nm) REMPI, and mass-selected ion imaging allows analysis of the velocity distribution of H-atoms from the HCCH hν→ C2H+H process. Measurement of the maximum velocity for H atoms from this channel produced by photodissociation of acetylene through the A˜ 1Au −X˜ 1∑+g V70K10, 110V40K10, 210V50K10 and V50K10 vibronic transitions gives a value for D00 (HCC–H) of 131±1 kcal/mol. Other channels producing hydrogen atoms (including HC2 hν→ C2+H and HCCHhν→ HCCH+ hν→ C2H++H) are detected at all photon fluxes used. These multiphoton channels produce hydrogen atoms with higher translational energy and therefore obscure measurement of the maximum velocity of H atoms produced by single-photon dissociation of acetylene. Reduction of photon flux by more than two orders of magnitude to ∼5×106 J/cm2 gives a background, multiphoton, H-atom intensity of ≤7% of the peak primary dissociation intensity. Because this multiphoton background limits the detectability of fast H atoms from single-photon dissociation of acetylene, the dissociation energy reported here is an upper limit. Calculations of potential rovibronic excitation of the C2H fragment are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458973
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  • 9
    Electronic Resource
    Electronic Resource
    Structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of CH2FCH2, CHF2CH2, and CF3CH2 radicals: An ab initio study (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6620-6629 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equilibrium geometries, rotational barriers, and harmonic vibrational frequencies for β-fluoroethyl (CH2FCH2), β,β-difluoroethyl (CHF2CH2), and β,β,β-trifluoroethyl (CF3CH2) radicals have been determined by ab initio molecular-orbital techniques using the gaussian 86 system of programs at the unrestricted Hartree–Fock, UHF/6-31G* level of theory. Three conformational minima in the potential-energy surface were found for the CH2FCH2 and CHF2CH2 radicals, while only one was found for CF3CH2. The radical centers for all three species are nonplanar, but the effect on the geometries of these radicals in replacing β-position hydrogen by fluorine atoms is much weaker than is the case for α-position fluorine substituted ethyl radicals. Transition structures for internal rotation were also located. Estimates for the correction of electron correlation effects were obtained by single-point calculations using second-order Moller–Plesset perturbation theory (UMP2). The rotation about the C–C bond is almost free for all three radicals, which again is much different from the situation in the α-fluorine substituted ethyl radicals. Based on calculated vibrational frequencies and moments of inertia, thermodynamic properties including heat capacities, entropies, enthalpy, and free-energy functions are tabulated as a function of temperature. Several isodesmic–homodesmic reactions have been studied for the purpose of obtaining theoretical heats of formation of the β-fluoroethyl and β, β-difluoroethyl radicals for which experimental values are not available. The theoretical heats of formation thus evaluated are −10.65 and −66.26 kcal/mol for CH2FCH2 and CHF2CH2, respectively. Together with the measured ΔH0f(CF3CH2), these data are used to evaluate ΔH0f,T, ΔG0f,T, and log Kf for all three radicals as a function of temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458929
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  • 10
    Electronic Resource
    Electronic Resource
    Distributed complex Gaussian basis sets: A useful function space for the solution of predissociation problems via the complex eigenvalue Schrödinger equation. Application to the isotope effect of NeH, NeD (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6642-6647 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method is presented for the solution of the complex eigenvalue Schrödinger equation for predissociation resonances, employing a basis set of complex Gaussians, with centers distributed along the internuclear axis. Application to the predissociation of the v=0 level of the A2Σ+ state of HeH, v=0–4 level of the A2Σ+ state of NeH and all the vibrational levels of he A2Σ+ state of NeD have yielded widths that are in very good agreement with the available experimental values and the recent observations [Devynck, Graham, and Peterson, J. Chem. Phys. 91, 6880 (1989)] of the strong suppression of predissociation in NeD as compared to that of NeH.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458932
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  • 11
    Electronic Resource
    Electronic Resource
    Geometries and energy separations of low-lying states of YNH and NYH (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6671-6675 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Complete active-space multiconfiguration self–consistent field followed by multireference configuration-interaction calculations are carried out on low-lying electronic states of YNH and NYH. We find the X 2Σ+ linear state of Y–N–H to be 55 kcal/mol more stable than the bent NYH and 59 kcal/mol more stable than the linear N–Y–H. Our calculations confirm the recent assignment of the first observed spectra generated by laser vaporization of Y metal + He/NH3. The theoretical dipole moment of the Y–N–H molecule (3.06 D) is in excellent agreement with an experimental value of 3.06 D obtained by Simard et al. The theoretical Y–N and N–H bond lengths are also in good agreement with the experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458935
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  • 12
    Electronic Resource
    Electronic Resource
    Approximate constants of motion for vibrational spectra of many-oscillator systems with multiple anharmonic resonances (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6630-6635 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory of approximate dynamical constants of motion is presented for vibrational (and implicitly, rovibrational) spectra of polyatomics with multiple nonlinear resonances. The formalism is developed in terms of simple vector algebra. The theory is applied to Hamiltonians used in fits of experimental spectra of H2O, CHClF2, and acetylene, with attention to reduced dimension motion, assignability of spectra, and statistical analysis of chaotic spectra. The approximate constants may be of interest as bottlenecks to vibrational energy flow in polyatomics. They may also be useful in reducing the size of basis sets in quantum-mechanical calculations of rotation–vibration spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458930
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  • 13
    Electronic Resource
    Electronic Resource
    Vibrations in small Mg clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6636-6641 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equilibrium geometries, binding energies, and harmonic frequencies of Mg3 and Mg4 have been determined using large atomic natural orbital basis sets in conjunction with high levels of electron correlation. The correlation treatments comprise multireference configuration interaction (MRCI), singles and doubles coupled-cluster (CCSD) theory and the CCSD(T) extension that includes a perturbational estimate of connected triple excitations. As with our previous studies of small Be clusters, the CCSD(T) method is found to reproduce the MRCI results with a remarkable degree of accuracy and at a fraction of the computational cost. Using the CCSD(T) method, full quartic force fields for Mg3 and Mg4 have been determined and anharmonic analyses have been performed using second-order perturbation theory. Vibration–rotation interaction constants and centrifugal distortion constants have also been determined. The fundamental vibrational frequencies are predicted to occur at ν1 (a'1)=96 and ν2 (e')=104 cm−1 for Mg3, and ν1 (a1)=184, ν2 (e)=143, and ν3 (t2)=167 cm−1 for Mg4. Infrared (IR) intensities, evaluated for the IR active modes via the double harmonic approximation, are 0.2 and 2.4 km/mol for the e' (Mg3) and t2 (Mg4) modes, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458931
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  • 14
    Electronic Resource
    Electronic Resource
    The role of lateral interactions in determining the properties of a dipolar monolayer at a polarizable interface (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6813-6819 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A statistical-mechanical description of a monolayer of solvent dipoles at a polarizable interface is presented in which the dipoles can take up three orientations with respect to the electrical field, namely, in its direction, against it, or perpendicular to it. The approach is based upon the Kikuchi method in which dipole–dipole interactions within an elementary cluster containing, in the present case, three molecules are treated exactly whereas those outside of the cluster are treated in the mean-field approximation. The differential capacity and monolayer entropy are calculated with consideration of lateral interactions between dipoles in the parallel orientation, and without these interactions. It is shown that the lateral interactions play an important role in determining monolayer properties near the zero charge potential. It is concluded that a more realistic description of such a monolayer can be obtained if the number of molecules in a basic cluster is significantly increased.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458951
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  • 15
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    Quantum statistical weights for reorientational correlation functions in the nuclear magnetic resonance relaxation of methyl groups (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6824-6833 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotationally symmetric groups containing nuclei with nonzero spin often exhibit uniquely quantum mechanical properties. Symmetry requirements on the total wave function restrict the allowable combinations of electronic, vibrational, rotational, and nuclear spin wave functions. NMR relaxation depends on operators that act on both the nuclear spin and rotational wave functions. The interdependence of the wave functions produces quantum statistical weights in the rotational correlation functions that can differ from semiclassical or classical probability distributions. The quantum statistical weights for the reorientational correlation functions in NMR relaxation have been derived for A3 and AX3 systems of spin 1/2 nuclei and for deuterium relaxation in a deuterated methyl or equivalent group. It is found that classical weights apply to deuterium relaxation. The A3 system has distinctly nonclassical weights. And the AX3 system has eight sets of weights, with different weights applying to different types of correlation functions. One of the eight is the classical weight, and two of the sets can lead to terms that do not arise in the usual, semiclassical theory. It is shown that, under certain circumstances, the predictions of this quantum mechanical theory differ from those of the semiclassical model. The present theory also predicts that changing the symmetry of the electronic/vibrational states can alter the NMR relaxation behavior. Calculations with a stochastic quantum dynamical model indicate that these differences may be experimentally important for, among other things, relaxation involving a methyl group in a biomolecule at physiological temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458927
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  • 16
    Electronic Resource
    Electronic Resource
    Unoccupied electronic structure of phthalocyanine films (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6859-6864 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The unoccupied molecular orbitals of metal-free phthalocyanine (H2PC), nickel-phthalocyanine (NiPC), and copper-phthalocyanine (CuPC) adsorbed on single crystal surfaces have been studied by inverse photoemission spectroscopy (IPES) and near-edge x-ray absorption spectroscopy (NEXAFS). The features observed in the experimental data are assigned by reference to molecular orbital calculations. Carbon and nitrogen 1s NEXAFS measurements of NiPC adsorbed on Cu(110) surface indicates that the phthalocyanine molecular planes are parallel to the surface. X-ray absorption data recorded at the Ni LIII and LII edges show that the unoccupied Ni 3d level is located in the plane of the phthalocyanine rings.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458918
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  • 17
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    Molecular photo cross sections with the LCGTO–Xα method using Stieltjes imaging (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5563-5572 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method for calculating and symmetry analyzing total molecular photoionization cross sections is presented. The technique is based on the LCGTO–Xα method and employs Stieltjes imaging. It allows applications to molecular systems comparable in size to those treated so far with the continuum multiple-scattering Xα method, but avoids the pitfalls of the muffin-tin approximation to the electronic potential. Photo cross sections for valence ionization of CO are found in good agreement with experiment and with previous calculations. From a final state symmetry analysis for the 1π level, the absence of kσ* shape resonance which appears in the 4σ and the 5σ ionization channels is attributed to small transition moments. This is in contrast to a previous treatment where this difference has been rationalized as caused by a channel dependent final state potential. The photoionization cross sections for the four highest valence orbitals of benzene were calculated in better agreement with experiment than found in a previous Xα–SW treatment. The improvement is especially significant for the 1e1g(π) highest occupied molecular orbital where at least part of the experimentally observed structures are attributed to shape resonances. Some of the observed resonance features in the valence orbital photo cross sections of benzene were identified with resonances found in carbon K-shell ionization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459626
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  • 18
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    Hund's coupling case sequences in resonant multiphoton transitions (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5370-5378 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Different Hund's coupling case sequences are considered for the n1+n2 near resonant multiphoton rovibronic process in electric dipole allowed transitions of any spin multiplicity. The transitional path interferences strength tensor is introduced. This tensor involves a polarization and rotational dependence as well as a transitional path dependence which couples the electronic vibrational motion with the rotational structure. The intensity of a rotational line may decompose in terms of the matrix element of this tensor and a pure electronic vibrational tensor. The specificity of the coupling case sequence is found condensed in the rotational line factors which are explicitly determined for all the coupling case sequences obtained from the case (a) and case (b) coupling.
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    URL: http://dx.doi.org/10.1063/1.459661
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  • 19
    Electronic Resource
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    Quasihomogeneous line broadening of a hydrogen-bonded polymer, investigated by picosecond infrared holeburning (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5365-5369 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transient spectral holes with lifetimes of several picoseconds are investigated in the infrared absorption band of the OH stretching vibration of the terpolymer polyvinyl butyral. A large variation of the quasihomogeneous linewidth from 7–45 cm−1 is determined from the measured holewidth in the temperature range 80–330 K and interpreted in terms of anharmonic coupling to a low frequency mode ν2. The result ν2=135±8 cm−1 provides strong evidence for indirect dephasing via the OH⋅⋅⋅O bridge stretching vibration. The inhomogeneous broadening of the total band is verified by the picosecond infrared holes. The inhomogeneity is related to an approximately static distribution of hydrogen bridges generated by local disorder and displays a different temperature behavior.
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    URL: http://dx.doi.org/10.1063/1.459660
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  • 20
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    Infrared laser spectroscopy of H2 and D2 Rydberg states. II. Diode laser spectra and assignment of 5g–4f, 6h–5g, and 8i–6h systems (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5417-5423 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Infrared diode laser absorption spectra of portions of the 5g–4f, 6h–5g, and 8i–6h Rydberg bands of H2 and D2 have been measured at Doppler limited resolution in low pressure A. C. discharges. The spectra, arising from L uncoupled states of H2 and D2, are assigned using an ab initio polarization model supported by intensity calculations. Details of the different implementations of this polarization model are given in the preceding paper. The most useful was the single channel vibrationally extended (1)/(2) V6 model which became progressively better at higher n (and L). Results of multichannel calculations for a selected set of transitions are also reported.
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    Asymmetric-to-centrosymmetric structure change of molecules in squaric acid crystal: Evidence for pressure-induced change of correlated proton potentials (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5429-5435 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-pressure effects on vibrational modes have been investigated in crystals of squaric acid H2C4O4 (H2SQ) and its deuterated analog (D2SQ) which show two-dimensional networks of hydrogen bonds. Specific vibrational modes in Raman and infrared spectra were observed to anomalously disappear with the application of pressures above 30 kbar in H2SQ, but not in D2SQ, indicating that the H2SQ molecule in crystal shows the asymmetric (C1h)-to-centrosymmetric (C4h) structure change. This is possibly caused by pressure-induced deformation of the correlated proton potential essentially from the double-well to single-well type.
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    URL: http://dx.doi.org/10.1063/1.459639
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    Brillouin scattering on the glass former GeSBr2 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5436-5447 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Brillouin scattering measurements were performed on the partly interrupted network glass former GeSBr2. Spectra were taken in symmetrical transmission arrangement, and in backscattering, thus varying momentum transfer between 2×104/cm and 44×104/cm. The temperature range was −100 °C〈T〈300 °C; the glass transition occurs at Tg∼−30 °C. The Brillouin spectra were carefully compared with viscoelastic theory. Strong relaxation accompanied by a dispersion of sound velocity occurs between 100 and 200 °C. A comparison of the spectra taken at different scattering angles and temperatures constitutes convincing evidence for a distribution of relaxation times (a Cole–Davidson distribution is used). The temperature dependence of the maximum relaxation time τ0 can well be described as Arrhenius-like, τ0 varying from 10−11 to〉5×10−9 s. The temperature dependences of the high frequency and the adiabatic sound velocities are derived. From the q-dependence, a fast relaxation channel can be identified. We discuss the assumption that it is directly connected with the break up of the glass above Tg. Including this term an agreement is reached between experiment and viscoelastic theory on the level of about 1% for the Brillouin line position and of about 10% for the linewidths, for the glassy, highly viscous, and (relatively) low viscosity regime.
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    Two dimensional 31P nuclear magnetic resonance coherence transfer experiments under magic-angle sample spinning (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5462-5472 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we report on two dimensional (2D) 31P cross polarization (CP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments on the coupled two-spin systems, sodium pyrophosphate decahydrate, Na4P2O7, 1OH2O (SP) and tetraphenyldiphosphine-1-oxide, (C6H5)2PP(O) (C6H5)2 (TPPO), including antiecho (COSY), double-quantum NMR, and zero-quantum NMR experiments. These experiments are generalizations of the absolute mode 2D Fourier transform antiecho COSY performed under MAS condition by Kentgens, de Boer, and Veeman [J. Chem. Phys. 87, 6859 (1987)]. The 2D sideband intensities for these experiments on polycrystalline samples are shown theoretically to be real. There are two mechanisms of coherence transfer; homonuclear J coupling and dipolar coupling. Theory shows that the zero-quantum signal for the coupled two spins can not be observed by using a (CPx−τ−(π)x−τ−(π/2)x−t1−(π/2)−t2 pulse sequence, when the coherence transfer is due to J coupling. When, however, the coherence transfer is induced by the flip-flop term of the dipolar coupling Hamiltonian, the zero-quantum signal can be observed by that pulse sequence. The preparation time dependences of the double-quantum and the zero-quantum sideband patterns, are expected, when the coherence transfer is induced by dipolar coupling. The zero-quantum signal was very weak for TPPO, while it was strong for SP. The apparent preparation time dependence of the zero-quantum sideband pattern was observed for SP. These results suggest that the coherence transfer is mainly due to J coupling in TPPO, where the two 31P nuclei have different isotropic chemical shifts. While, on the other hand, the dipolar coupling is more important in SP, where the two 31P nuclei have the same isotropic chemical shifts but different orientations of the chemical shift tensors. The 2D sideband intensities of the antiecho COSY spectrum of TPPO were calculated, and the relative orientation of the two chemical shift tensors was determined.
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    Visible and uv spectra of a polydiacetylene with a side group conjugated to the main chain (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5510-5517 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The polarized reflection spectra of single crystals of PDA-CPDO [poly-1-(N-carbazolyl)penta-1, 3-diyn-5-ol], which are π conjugated between the side groups and the main chain, have been measured in the photon energy region from 1.38 to 32 eV for the first time with a polarized synchrotron radiation source. Absorption spectra have been calculated using the Kramers–Kronig relation. Transitions in the visible absorption spectrum, which have been previously attributed to an interband transition in a similar polydiacetylene, have been found to be highly dichroic with respect to the direction along the polymer backbone. The absorption spectrum in this region reveals two broad (ΔE(approximately-greater-than)0.4 eV) peaks at 1.9 and 2.7 eV with an almost equal absorption coefficient of 7.5×104 cm−1. The spectral features in the range from 3 to 8 eV result from electronic transitions of the carbazolyl side groups. A single broad (ΔE∼10 eV) absorption band observed at 18 eV is due to either transitions of σ electrons to higher σ* or π* states of the carbazolyl group, or to ionization processes.
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    A centrifugal-sudden distorted-wave study of isotope effects for the reactions Cl+HCl→ClH+Cl and Cl+DCl→ClD+Cl (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5544-5551 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A quantum calculation has been performed using the centrifugal-sudden distorted-wave (CSDW) method for the three-dimensional Cl+DCl→ClD+Cl reaction. Three potential energy surfaces have been employed: two extended London–Eyring–Polanyi–Sato surfaces [denoted Bondi–Connor–Manz–Römelt (BCMR) and Persky–Kornweitz 3 (PK3)] and a scaled and fitted ab initio one (denoted sf-POLCI). Quantities calculated include: cumulative reaction probabilities, integral cross sections, rotational product distributions, and rate coefficients. Differential cross reactions are also reported for the PK3 surface, which are compared with the results from a simple semiclassical optical model (close agreement is found). We also compare the Cl+DCl results with earlier CSDW calculations for Cl+HCl→ClH+Cl. The rotational distributions are strongly perturbed by isotope substitution and are sensitive to variations in the potential surface. In contrast, the H and D rate coefficients for all three surfaces agree with experimental values, except for Cl+DCl on PK3.
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    URL: http://dx.doi.org/10.1063/1.459624
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    An energy functional of electron-pair density (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5856-5861 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electron-pair (or intracule) density is the probability density function for an interelectronic vector and is intimately related to the electron correlation in many-electron systems. Based on the local scaling method, a theory is presented for the direct variational determination of the electron-pair density. Illustrative applications are given for the ground state of the helium atom. Simple electron-pair density functions are reported which compare quantitatively with the exact density.
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    On the electronic structure of electrides (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5875-5882 
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    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We explore a mechanism for the remarkable charge isolation of the localized, or trapped, electrons found in the crystalline electrides Cs(18C6)2 and Cs(15C5)2. 133Cs NMR measurements show only ≈ 0.05% atomic character of the spin density at the Cs nucleus, consistent with many features of the structure and measured properties which indicate that the localized electron distribution is centered at the anion vacancies. The optical absorption data suggest that the localized electrons, which give rise to the Curie-law spin susceptibility, must penetrate appreciably into the crown ethers, (18C6) and (15C5), which encapsulate the Cs. We suggest that the large reduction of the spin density at the Cs nucleus is due to a Coulomb barrier resulting from negative charge on the oxygens. A crude model, one electron moving in two spherical charged shells surrounding the Cs core, illustrates the mechanism and accounts accurately for the ratio of spin densities at the Cs nucleus found in the 18C6 and the 15C5 electrides. Hartree–Fock calculations for an idealized model of an isolated Cs(18C6)2 molecule, namely Li(9C3)2, tend to support the mechanism.
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    Hund's case (e): Application to Rydberg states with a 2Π ionic core (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5898-5901 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Rydberg states of high n belonging to series converging to an ion state with a nonzero spin–orbit splitting must be described in so-called Hund's case (e). The derivation of the wave function obtained by transition from case (a) to case (e) is given. Application is made for a 2Π ionic core. This scheme is particularly useful to understand, for example, rotationally resolved spin–orbit autoionization.
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    A transition state theory-based statistical distribution of unimolecular decay rates with application to unimolecular decomposition of formaldehyde (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5657-5666 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A statistical distribution of state-specific unimolecular decay rates is derived (within the framework of random matrix theory) that is determined completely by the transition state properties of the potential energy surface. It includes the standard χ-square distributions as a special case. Model calculations are presented to show the extent to which it can differ from the χ-square distribution, and specific application is made to the state-specific unimolecular decay rate data for D2CO→D2+CO.
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    Where does overtone intensity come from? (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6140-6147 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The factors that influence overtone intensity are examined using realistic potential and dipole moment functions that model the acetylenic stretching vibration. It is found that mechanical anharmonicity is more important than electrical anharmonicity, especially at higher overtone levels but a quantitative treatment requires both contributions which are not simply additive. It is found numerically that the inner wall of the potential dominates in determining overtone intensity. This can be justified by heuristic arguments and by the properties of the sensitivity functional of the transition dipole matrix elements to changes in V(r). It is suggested that absolute overtone intensities could be useful in determining the inner walls of hydrogen stretching vibrations.
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    The size distribution of rigid segments in a one-dimensional equilibrium fluid (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6003-6005 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The possibility of obtaining information on the size distribution of a rigid segment 1D fluid from its experimentally determined radial distribution function, subject to random noise, was investigated. It was found that a direct determination of the size distributions is impossible since the solutions are unstable even for very low levels of noise. Using the moments of the radial distribution function, however, it is possible to obtain reliable data on the density and packing fraction of the fluid and on the average size and dispersion of the size distribution, and good information on its asymmetry parameter and kurtosis, which may allow a distinction between different shapes.
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    Computational study of transition dynamics in 55-atom clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6013-6024 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics computer simulation has ben employed to study structure and isomerization dynamics of intact 55-atom clusters. The interactions used were selected to represent the heavier noble gases Ar, Kr, and Xe. As an aid for interpretation of results, the molecular dynamics computation was coupled to steepest-descent mapping to locate relevant cluster inherent structures (potential energy minima). A relatively sharp melting transition has been reproducibly observed. In its low-temperature "solid state'' the cluster predominately inhabits the basins for the Mackay icosahedral inherent structure, with occasional excursions into and out of particle–hole states (an atom promoted from filled second to empty third icosahedral shell). Most inherent structures for the liquid droplet state are amorphous, are higher in energy than those for the solid, have no obvious icosahedral ancestry, and display surface capillary excitations. Freezing can produce defective solid structures which then can be annealed to the ground-state icosahedral structure. Root-mean-square distances under mapping to minima have been evaluated vs temperature; they show behavior qualitatively similar to, but quantitatively shifted from, the bulk-phase behavior prescribed by the Lindemann melting criterion and its conjugate freezing criterion.
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    A molecular dynamics study of ion bombardment induced ejection of C2H4 from Ni(111): Relationships of the angle distributions of the translational and rotational angular momentum of ejected C2H4 to surface structure (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6041-6046 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The angular distributions of the translational and rotational angular momentum of C2H4 molecules ejected from a Ni(111) surface by ion bombardment have been calculated from molecular dynamics simulations of the ion–surface collision process. Both the translational and rotational momentum of ejected C2H4 tend to orient in preferred directions which are dependent on the C2H4 bonding site geometry. Most C2H4 molecules are ejected by a collision with a single energetic Ni atom. It is shown that the translational angle distribution is governed primarily by the distribution of Ni–C2H4 collision geometries. A preference for Ni atoms to eject in the directions between surrounding nearest neighbor Ni atoms gives rise to preferred Ni–C2H4 collision geometries that cause C2H4 to eject in preferred directions. The preferred orientations of the rotational angular momentum of ejected C2H4 are shown to be governed directly by the orientation of the C–C bond axis of the original chemisorbed C2H4 molecules, independent of the Ni–C2H4 collision geometry.
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    Analysis of a model for the ripple phase of lipid bilayers (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6070-6080 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report two-dimensional Monte Carlo, mean-field, and exact one-dimensional studies of a statistical-mechanical model for the ripple (Pβ') phase of hydrated phosphatidylcholine lipid bilayers. The model is a p-state chiral clock model coupled to an Ising model through the chiral field. The microscopic parameters of the model are fixed by independently obtained values for pairwise molecular interactions. The one-dimensional model possesses a "floating fluid phase'' characterized by exponentially decaying, spatially modulated correlations. Solutions of the mean-field equations for the model include modulated phases but these are found to be metastable states, and the mean-field phase diagram is dominated by phases characteristic of the p-state chiral clock model. Monte Carlo simulations also reveal clock model phases. However, when unphysical effects due to mod p counting in the Hamiltonian are eliminated, the Monte Carlo simulations reveal a modulated phase, intermediate in temperature between a high-temperature disordered phase and a low-temperature ordered phase which is identified with the chain tilt (Lβ') phase.
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    Excimer emission of molecular systems with energy transfer (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4516-4519 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The exact solution of kinetic equations of a simple chain of molecular excimer forming system is derived. The influence of energy transfer on both monomer and excimer emission kinetic is discussed.
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    Excited-state proton transfer in gas-phase clusters: 2-Naphthol⋅(NH3)n (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4520-4532 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Excited-state proton transfer was studied in supersonically cooled neutral acid-base clusters of 2-naphthol⋅(NH3)n with n=1-10, using size-selective two-color resonant two-photon ionization (R2PI) and fluorescence emission techniques. The smaller clusters (n≤3) show vibrationally resolved spectra and do not exhibit excited-state proton transfer. Two rotamers (cis and trans) were observed for each cluster size; these exhibit very different electronic-vibrational couplings to the intermolecular modes, reflecting the orientation of the (NH3)n cluster relative to the electronic transition dipole moment of the aromatic chromophore. Excited-state proton transfer occurs for n≥4, as evidenced by (a) a large spectral red shift (≈8000 cm−1) of the fluorescence emission band; (b) large increase in the emission bandwidth; (c) similarity of the fluorescence band position and width to that of 2-naphtholate anion in neutral or basic aqueous solution. In absorption, the S1←S0 bands are substantially broadened for clusters with n≥4, but are not shifted to the red, as is the case for the analogous 1-naphthol⋅(NH3)n clusters. The cluster-size dependence of the proton transfer reaction is mainly controlled by the cluster proton affinity.
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    Torsional band assignment and intramolecular twist potential of 9,9'-bianthryl and its 10-cyano derivative in a free jet (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4543-4555 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a pulsed supersonic beam and laser-induced fluorescence spectroscopy the torsional structure of the S0 and S1 states of 9,9'-bianthryl (BA) and its derivative 10-(cyano)-9,9'-bianthryl (CBA) was investigated. Taking into account the very different equilibrium positions of the S0 and S1 potentials which result in a nonobservable 0–0 transition within the jet spectra, a new and straightforward procedure of torsional band assignment is carried out. This is based on a characteristic pattern of Franck–Condon factors within the dispersed fluorescence spectra. The torsional potentials were determined by a fit procedure of a one-dimensional model to the experimental data. The results show that the S1 double minimum potential for BA is shallower than for CBA indicating a stronger interaction between the molecular halfs of the latter compound. The observed rotational contours of torsional bands recorded for CBA reflect the change from a symmetric top molecule (for states above the S1 torsional barrier) to an asymmetric top (for states below the barrier) and manifest the tunneling splitting of the level just below the barrier. The dispersed fluorescence spectra of CBA are discussed in terms of intramolecular vibrational redistribution (IVR) processes. The measured fluorescence decay rates as a function of excess vibrational energy of CBA reflect a saturation behavior already within the origin region in contrast to BA (saturation near 380 cm−1). This is tentatively ascribed to a low lying dark background state possibly of charge transfer character.
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    Infrared spectroscopy and ab initio theory of the structural isomers of CO2–HCN (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4560-4570 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Infrared spectra have been obtained for the liner and T-shaped isomers of CO2–HCN using the optothermal detection method. These spectra correspond to the intramolecular C–H stretching vibration in each isomer. The infrared results are consistent with the structures determined previously from microwave spectroscopy and provide additional data that is helpful in making meaningful comparisons with theory. Ab initio calculations have also been carried out for this system using three different basis sets. The agreement between the experimental and calculated structures is good for all three basis sets, while the vibrational frequency shifts (scaled to the monomer) and the intermolecular stretching force constants are much more basis set dependent. Nevertheless, the overall agreement between experiment and ab initio theory is very good.
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    High-resolution rotation–vibration spectroscopy of difluorophosphorane: A combined theoretical and experimental study (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4603-4614 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The harmonic and anharmonic force fields of PH3F2 have been calculated at the ab initio self-consistent-field level using polarized split-valence and triple-zeta basis sets. PH3F2 has been prepared in pure form and identified unambiguously in the gas phase by Doppler-limited Fourier transform infrared spectroscopy. Guided by the theoretical predictions for the rotation–vibration spectra and the spectroscopic constants, the observed bands have been assigned and several of them rotationally analyzed, in particular ν4/2ν±27, ν2+ν4/ν2+2ν±27, and ν5/ν3+ν8. PH3F2 is found to have a D3h structure, ro(PH)=1.394(4) A(ring) and ro(PF)=1.6468(2) A(ring). The theoretical results are in good agreement with experiment, both for the structure and for the available spectroscopic constants. The present study demonstrates the advantages of a combined theoretical and experimental approach to the spectroscopy of reactive molecules.
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    Intensities in local mode overtone spectra: Propane (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6239-6248 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The gas phase vibrational overtone spectrum of propane is measured using conventional near infrared (NIR) spectroscopy for the ΔvCH=2–5 regions and intracavity dye laser photoacoustic spectroscopy (IDL-PAS) for the ΔvCH=5 and 6 regions. The peaks are assigned in terms of the local mode model. Experimental oscillator strengths are compared to values calculated for the CH-stretching components of the spectrum. The calculations use a harmonically coupled, anharmonic oscillator local mode model to obtain the vibrational wave functions, and ab initio MO calculations at the SCF level with a 6-31G* basis set to obtain the dipole moment function. The importance of intermanifold coupling is explored. The calculations can account for the fall-off in intensity with increasing v, and can give a reasonably quantitative account of the relative intensities of the individual peaks within a given vibrational manifold. The questions of the relative intensities of primary and secondary CH bonds, and of the relative intensities of different methyl CH bonds are also explored.
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    On the nature of the orientational transition in crystalline A–TCNB (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5935-5939 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientational properties of crystalline A–TCNB are calculated using a Monte Carlo procedure, based upon a previously reported site–site expression for the interactions between molecules. The nearly static TCNB complexes are orientationally quiescent. The A molecules respond to an orientational double well potential provided by the TCNB, and by their mutual interactions. As a consequence, the dominant part of the orientational Hamiltonian depends on only a single degree of freedom, from which the results are determined. Properties calculated include the orientational structures, internal energy, specific heat, and the phase transition temperature. Various order parameters are calculated to understand the microscopic nature of the transition, which seems to show features of both an order–disorder and displacive character, much like experimental interpretations.
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    Dynamics of an ammonium ion in water: Molecular dynamics simulation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5961-5966 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The translational and rotational dynamics of an aqueous ammonium ion are examined using an NVE molecular dynamics computer simulation and a rigid model of water. The linear momentum, angular momentum and reorientational autocorrelation functions of the ammonium ion have been measured. From these, the memory functions for the linear and angular momentum correlation functions are calculated. Reorientation of the ammonium ion about its center of mass shows unambiguous departure from Debye rotational behavior. The translational and rotational diffusion constants have been calculated.
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    Thermochemistry of the gaseous thorium bromides (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5983-5989 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The sublimation of ThBr4(s) was studied by the torsion-effusion and mass spectrometric methods in the range 600–750 K; only monomeric ThBr4 vapor was observed, and the pertinent thermodynamic parameters were derived. The gaseous lower-valent species ThBr3, ThBr2, and ThBr were identified in the bromination of Th(s) at 1500 to 2000 K, and several reaction equilibria involving these species were studied by mass spectrometry over wide temperature ranges for second law analysis. Reaction enthalpies, bond dissociation energies, and standard enthalpies of formation were derived from the results. Unlike ThF4 and ThCl4, the sublimation entropy indicates that ThBr4 may have a distorted tetrahedral structure. Bond dissociation energies and related thermodynamic properties are discussed and compared with values in other thorium and uranium halides.
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    Computer simulation study of the conformational statistics of long chain hydrocarbons in cylindrical potentials (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5990-5997 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Monte Carlo method is applied to the study of disordered conformations of polymethylene chains confined in cylindrical mean-field potentials. It is assumed that the molecule, which is composed of 30 united atoms (methylene groups), has fixed bond length and bond angle, and makes quasicontinuous bond rotations. Various statistical properties of the molecule, such as dihedral angle distributions, dihedral angle pair correlations, transverse fluctuations, etc., are calculated vs strength of the mean-field potential. The dihedral angle distributions calculated exhibit the marked reduction of the gauche peaks with increasing potential; it implies the increasing inaccuracy of the usual rotational isomeric model. The dihedral angle pair correlations reveal novel characteristics of the dihedral angle fluctuation: the fluctuation has approximate period of four bonds with marked tendency for the next nearest bonds to counter-rotate. The characteristics are more conspicuous under weaker potential constraint. There are large transverse fluctuations of the chain, the average linear form of the chain being still maintained. These characteristic dihedral angle fluctuation and the transverse deviation of the chain are found to be well understood by a small scale kink model.
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    Transport properties of polydisperse fluids. II. Effect of solute polydispersity on the bulk and shear viscosities of a solute–solvent mixture of hard spheres in the Enskog approximation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6025-6033 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The bulk and shear viscosity expressions for a binary mixture of hard spheres in the Enskog transport theory are generalized to the case in which one of the species (the solute) becomes polydisperse. The effect of polydispersity is expressed in terms of correction factors to results for a binary mixture with a monodisperse solute. These correction factors are evaluated for both a log-normal size distribution of solute particles with the solute mass-size relation of power-law form and a log-normal mass distribution of solute particles with fixed solute-particle size. The results reported here can be easily generalized to a multicomponent mixture involving several polydisperse species and several monodisperse species.
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    Comparison of light scattering of colloidal dispersions with direct force measurements between analogous macroscopic surfaces (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6057-6064 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equilibrium force laws (force vs distance) between two macroscopic surfactant-coated mica surfaces separated by organic liquids have been determined. The aim was to ascertain whether directly measured forces are able to be employed in the prediction of second virial coefficients as obtained from light scattering experiments on analogous colloidal suspensions (i.e., spherical particles coated with the same surfactant and dispersed in the same medium). The quantitative agreement between the two independent measurements was excellent. This suggests that the interaction energy per unit area, which governs the physical properties of a bulk colloidal system, is indeed, the same as the equilibrium interaction energy per unit area of two macroscopic surfaces, for the systems investigated here. Such a correlation has been tacitly assumed in the past, and it is the first time that this assumption has been confirmed experimentally for any system.
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    Experimental determination of thermal and nonthermal mechanisms for laser desorption from thin metal films (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4719-4723 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The mechanism of laser desorption of peptides as negative ions from Au thin films with 193, 248 and 351 nm laser radiation has been studied. Variation of the threshold laser power density with metal film thickness is used to distinguish between thermal and nonthermal mechanisms. The influence of laser wavelength on the desorption of peptides with different optical absorption spectra has been studied. Thermal desorption is observed when 351 nm laser radiation is utilized. However, both 248 and 193 nm radiation result in nonthermal desorption processes. At 248 nm, the threshold power density is observed to be independent of the optical absorption of the peptide adsorbate, supporting suggestions that a mechanism involving excitation of hot electrons in the metal is important.
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    An interpretation of molecular fragmentation dynamics in terms of the resonance spectrum (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4750-4760 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quasidegenerate perturbation theory using a Bloch wave operator formalism is applied to the calculation of the rich resonance spectrum of a model polyatomic molecule undergoing photodissociation. The sharp structures (peaks and dips patterns) of the total dissociation rate as well as the partial probabilities to obtain the fragments at given internal states for various incident photon energies, are interpreted in terms of tunneling, shape or Feshbach resonances.
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    Brute force in molecular reaction dynamics: A novel technique for measuring steric effects (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4779-4790 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the theoretical foundation of a novel technique for the orientation of ∑-molecules and symmetric tops with permanent electric dipole moment. The method bases on the adiabatic transformation of free rotational states into those of librational oscillations taking place during the passage of a molecule into a strong electric field. Several examples of calculated spatial distributions of the molecular axis are given. The computational results demonstrate that in connection with highly relaxed supersonic nozzle beams only moderately strong electric fields are required to generate a marked orientation of the axis. In a first application of this technique we studied steric effects of the reaction K+CH3I→KI+CH3 at two elevated collision energies (0.79 and 1.24 eV). All observed steric effects could be rationalized in terms of a simple impulsive reaction model. We find that backward scattered products are more likely formed if the approaching atoms face the I-end of CH3I—in agreement with low energy results—while forward scattered products are favorably formed if the CH3-end is encountered.
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    Proton spin–lattice relaxation rate in supercooled H2O and H2 17O under high pressure (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4796-4803 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The proton spin–lattice relaxation rate has been investigated as a function of pressure (p≤250 MPa), temperature (180 K≤T≤300 K), and oxygen-17 enrichment (25.7 atom-%, 50.7 atom-%) in metastable water. Intramolecular proton–oxygen 17 dipolar interactions in light water in relation to deuterium electric quadrupole interactions in heavy water carry similar dynamic information and allow the complicated dynamic isotope effect in liquid water to be investigated in the whole metastable range. Under hydrostatic pressure the proton relaxation rate curve R1 (T, p, ωH) has been investigated at five different Larmor frequencies (100 MHz≤ω≤500 MHz). These experiments unequivocally prove the isotropy of reorientational motions of water molecules on a nanosecond time scale and demonstrate the necessity to include fast quasilattice vibrations into a consistent interpretation of the relaxation rates in the dispersion regime.
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    Theoretical and experimental total state-selected and state-to-state cross sections. III. The (Ar+H2)+ system (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4845-4857 
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    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A detailed three-dimensional quantum mechanical study of the (Ar+H2)+ system along the energy range 0.4 eV≤Etot≤1.65 eV is presented. The main difference between this new treatment and the previously published one [J. Chem. Phys. 87, 465 (1987)] is the employment of a new version of the reactive infinite-order sudden approximation (IOSA), which is based on the ordinary inelastic IOSA carried out for an optical potential. In the numerical treatment we include three surfaces (only two were included in the previous treatment), one which correlates with the Ar+H+2 system and two which correlate with the two spin states of Ar+(2Pj); j=3/2,1/2. The results are compared with both trajectory-surface-hopping calculations and with experiments. In most cases, very good agreement is obtained.
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    Scaling the reaction path potential (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4938-4945 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A molecular potential energy surface may be approximately described in terms of the energy, energy gradients, and second derivatives at a discrete set of points along a reaction path. The accuracy of this approach is limited, in part, by the level of ab initio theory that can be employed to obtain energy gradients and second derivatives. A method is derived and demonstrated whereby the energy surface can be scaled to achieve a greater degree of accuracy, using a set of energies calculated at a higher level of ab initio theory.
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    On the infrared spectroscopy of SiF4 and SF6 in Ar clusters: Location of the solute (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4898-4906 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the infrared photodissociation spectra of argon clusters containing SiF4, as a function of the cluster size n (for n≤ 103) using molecular beam laser spectroscopy. The clusters were produced by both the conventional seeded expansion of a dilute mixture and by a "pickup'' method where, upon colliding with it, the chromophore sticks to the surface of a cluster made in a neat solvent expansion. Furthermore, the spectra of small SF6/Arn clusters (n≤50) have been remeasured with the improved resolution resulting from the use of two line and tunable isotopic CO2 lasers. These data, together with previously published data on SF6/Ar, indicate a remarkably different behavior for these two solute–solvent combinations. The preferred "site'' for SiF4 is at the surface of Ar clusters of all sizes, regardless of how the molecule is introduced to the cluster, while appreciable amounts of SF6 reside at the surface only when the cluster is large and the impurity is deposited onto the cluster surface. The behavior of SiF4 and SF6, together with the analogous behavior of other polyatomic chromophores, the IR spectra of which have been measured and reported previously [D. J. Levandier, M. Mengel, R. Pursel, J. McCombie, and G. Scoles, Z. Phys. D 10, 337 (1988); D. J. Levandier, S. Goyal, J. McCombie, B. Pate, and G. Scoles, J. Chem. Soc. Faraday Trans. 86, 2361 (1990)], can be rationalized in terms of molecular dynamics simulations of similar systems which are presented in the paper by Perera and Amar [L. Perera and F. G. Amar, J. Chem. Phys. 93, 4884 (1990)]. The combination of the theoretical and experimental results confirm the usefulness of infrared photodissociation spectroscopy for the study of the structure of clusters and suggest that assuming any particular location for an impurity in a cluster in the absence of experimental evidence or, at least, a dynamics calculation, can easily lead to wrong conclusions.
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    The effects of core (3d) correlation on chemisorption (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4954-4957 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Large-scale calculations using atomic natural orbital (ANO) basis sets have been performed on Cu5O to establish the effects of correlation of the 3d shell on oxygen chemisorption. The largest calculation performed correlated 51 electrons in a basis set containing 205 ANO's. Correlation of the 3d shell is found to increase the chemisorption energy by 13(16±2) kcal/mol and decrease the height above the surface of the oxygen by 0.23(0.38) a0 with CI superposition error corrections included. The 2E state of the bare cluster is found to be stabilized by 10(8) kcal/mol relative to the 4A2 state as a result of 3d correlation. The values in parentheses were obtained using an approximate core–valence correlation operator which thus gives results in very good agreement with the core correlated calculations.
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    Weak predissociation of the EF, GK, and H 1Σ+g states of the H2 molecule by nonadiabatic coupling with the electronic ground state (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4958-4964 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energy shifts and widths generated in the vibrational levels of the EF, GK, and H 1Σ+g states of H2 by their nonadiabatic interactions with the electronic ground state have been calculated using ab initio wave functions. The shifts of the 44 quasibound vibronic levels of these excited states range from +0.02 to +1.00 cm−1. Along the EF progression the shifts are found to depend on vibrational kinetic energy in surprisingly good accord with the Van Vleck approximation. The calculated nonradiative decay probabilities range up to 3.3×106 s−1, and in one case the predissociation rate is as large as 39% of the radiative one. Experimental tests of these predictions are not yet available.
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    Rotational spectra and structures of small clusters containing the HCN dimer: X–(HCN)2 with X=CO, N2, NH3, and H2O (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6363-6370 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This work is the counterpart of a previous report on the (HCN)2–Y trimers with Y=HF, HCl, HCF3, and CO2 [J. Chem. Phys. 90, 4069 (1989)]. Rotational spectra have been observed for several isotopic species of the OC–, N2–, H3N–, and H2O–(HCN)2 trimers, using a pulsed nozzle, Fourier transform Balle/Flygare microwave spectrometer. The structures are basically composites of those reported for the (HCN)2 and X–HCN dimers. The trimers are effectively axially symmetric, but have some shrinkage of dimensions. Rotational constants found for the main isotopic species of each trimer are: For X=OC, a B0 of 421.142 MHz and DJ of 110 Hz; for X=N2, 435.573 MHz and 155 Hz; for X=H3N, a symmetric top, a B0 of 675.777 MHz, DJ of 180 Hz, and DJK of 41.1 kHz; and for X=H2O, with C2v symmetry, a (B0+C0)/2 of 667.028 MHz, (B0−C0)/2 of 0.617 MHz, DJ of 173 Hz, and a DJK of 62.9 kHz. The rotational constants for the isotopic species of each trimer were used to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers, r1 being that for X–HCN and r2 that for (HCN)2. For X=OC, N2, H3N, and H2O the shrinkages found in r1 are 0.068, 0.056, 0.084, and 0.074 A(ring), respectively, and in r2 0.013, 0.013, 0.044, and 0.026 A(ring). The 14N quadrupole coupling constants were determined by selective 15N substitution for most of the nitrogen sites in the trimers. The effects of charge redistribution in the trimers were separated from those of torsional oscillations in several instances including N2 in N2–(HCN)2.
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    The hydrogen atom channel in the photodissociation of ethylene (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6398-6402 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of ethylene at 193 nm is studied by laser induced fluorescence of the hydrogen atom product. The quantum yield for producing deuterium atoms from C2D4 is 0.19±0.04. Their average translational energy is 7.3±1.3 and 15.1±1.4 kcal/mol at 193 and 157 nm, respectively. Considerable evidence, both experimental and theoretical, shows that dissociation takes place from a hot internally equilibrated ground state molecule. A remarkable isotope effect is found. The ratio of H atoms to D atoms released from CH2CD2 is 2.2±0.2, whereas that from t-CHDCHD is 1.2±0.1. An RRKM model explains these ratios. For CH2CD2 the vibrational frequencies of the transition state for releasing H atoms are quite different from those for the release of D atoms. In t-CHDCHD, the transition state frequencies are nearly the same; there is a cancellation between differing zero point energies and moments of inertia.
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    An approach to the "channel three'' phenomenon of benzene (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6433-6439 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The nonradiative decay of benzene excited within the S1 manifold is studied by assuming an open vibrational potential for the S2 state that leads to spontaneous breaking of hexagonal symmetry along a path for valence isomerization. The S2 state crosses the S1 electronic configuration (at the channel three threshold) as well as the ground state along the reaction path. The rate constants of nonradiative decay of the S1 state (in fact the S1uS2 internal conversion) and the S2 state (S2uS0 IC) are both quantum mechanically calculated by invoking microcanonical averaging over all vibrational levels accessible at a given excess of vibrational energy. The results obtained account for a number of features observed in spectroscopy of benzene at the channel three threshold.
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    Role of photoacoustics in optogalvanics (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3899-3906 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Time-resolved laser optogalvanic (LOG) signals have been induced by pulsed laser excitation (lsj→2pk, Paschen notation) of a ∼30 MHz radio-frequency (rf) discharge in neon at ∼5 torr. Dramatic changes of the shape/polarity of certain parts of the LOG signals occur when the rf excitation frequency is scanned over the electrical resonance peak of the plasma and the associated driving/detecting circuits. These effects are attributed to ionization rate changes (i.e., laser-induced alterations of the plasma conductivity), with concomitant variations in the plasma resonance characteristics. In addition to ionization rate changes, it is shown that photoacoustic (PA) effects also play a significant role in the generation of the LOG signal. Those parts of the LOG signal that are invariant with respect to the rf frequency are attributed to a PA effect. The similarity of LOG signal shapes from both rf and dc discharges suggests that photoacoustics play a similar role in the LOG effect in dc discharges. Contrary to common belief, most reported LOG signal profiles, ones produced by excitation to levels that do not lie close to the ionization threshold, appear to be totally mediated by the PA effect.
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    A new computational approach to the chemical potential (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5167-5171 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new approach to the computation of chemical potential and free energy is presented. The basic idea is to correct the underestimate obtained by the particle-removal method. Application of the new technique to several different thermodynamic states of the Lennard-Jones (12,6) fluid shows that it is capable of providing reliable estimates of the chemical potential, even at high density where standard methods encounter computational difficulties.
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    Langevin dynamics study of NaCl electrolyte solutions at different concentrations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5177-5181 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Langevin dynamics simulations of NaCl electrolyte solutions at 25° C and several concentrations (0.1–2 M) were performed. The solvent-averaged potentials of Pettitt and Rossky were assumed. These potential models show remarkable oscillatory shapes that are associated with the discrete structure of the solvent. The changes in the ionic structure and self-diffusion coefficients with the increase of the concentration are discussed. Special attention is paid to both the mean number of ionic pairs and the time that these ionic associations survive. The results are compared with the ones obtained from other potential models and with the available experimental data.
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    Equilibrium and nonequilibrium solvation and solute electronic structure. I. Formulation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5194-5210 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical formulation is developed to describe the electronic structure of an immersed solute, electrostatically coupled to a polar and polarizable solvent. The solvent is characterized, in the dielectric continuum approximation, by electronic and orientational polarizations. Starting from a general free-energy expression for the quantum solute–solvent system, a time-independent nonlinear Schrödinger equation is derived. The nonlinearity arises from the assumed equilibration of the solvent electronic polarization Peqe, to the solute electronic wave function Ψ and the solvent orientational polarization Por. When Por is arbitrary, there is nonequilibrium solvation. When Por is equilibrated to Peqe and Ψ, equilibrium solvation obtains. The theory is illustrated for a model symmetric electron donor–acceptor solute system in a two state basis set description. Solution of the nonlinear Schrödinger equation in the presence of arbitrary Por yields nonequilibrium solvation stationary states (NSS) for the solute–solvent system, including the solvent-dependent solute electronic structure, and the associated free energies. When Por=Peqor, the corresponding equilibrium solvation states (ESS) and their characteristics are obtained. The NSS are classified into three distinct regimes, according to the relative strengths of the electronic coupling, which tends to delocalize the solute electronic distribution, and the solvent polarization, which tends to localize it. The ESS stability characteristics are also important in this classification. Two of the regimes correspond to activated electron transfer processes, and differ according to whether there is or is not a continuous free-energy path leading from localized reactants to localized products. The third regime, in which the electronic coupling dominates the solvent polarization, corresponds to stable delocalized states between which spectroscopic transitions are of interest. Finally, the inclusion of electronic exchange in the theory leads to the necessity of more than one solvent coordinate in order to describe the free-energy surface for the solute–solvent system.
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    Molecular-dynamics simulation of aqueous mixtures: Methanol, acetone, and ammonia (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5156-5166 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Effective pair-potential models, parametrized to the properties of the pure liquids, have been used in molecular-dynamics simulations of aqueous (binary) mixtures containing methanol, ammonia, or acetone. Results are reported for thermodynamic and structural properties, self-diffusion coefficients, and reorientational correlation times. There is fair agreement with a wide variety of experimental data. The pattern of hydrogen bonding and the distribution of hydrogen-bond lifetimes in the simulated mixtures have been investigated. The observed anomalous behavior of methanol and acetone solutions appears to be related to specific features of the hydrogen bonding—namely, the ability of these molecules to exhibit enhanced acceptor character. As a consequence of the assumed intermolecular potentials, the balance between the competing effects of hydrophobic hydration of methyl groups and hydrogen bonding to oxygen atoms is tipped towards the latter. A number of interesting structural effects have been noted. In particular, the distribution function g(R) for water oxygen atoms sharpens appreciably on addition of the second component. The structure of the water–ammonia system is discussed in detail. There is good accord with available x-ray data at low concentration, but there are no measurements available to test the predictions at high concentration.
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    Freezing of simple systems using density functional theory (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5187-5193 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Density functional theory (DFT) has been applied to the study of the fluid–solid transition in systems with realistic potentials (soft cores and attractive forces): the purely repulsive WCA Lennard-Jones reference potential (LJT), the full Lennard-Jones potential (LJ) and the exponential-6 potential appropriate for helium and hydrogen. Three different DFT formalisms were used: the formulation of Haymet and Oxtoby (HO) and the new theories of Denton and Ashcroft (MWDA) and of Baus (MELA). The results for the melting pressure are compared with recent simulation and experimental data. The results of the HO version are always too high, the deviation increasing when going from the repulsive Lennard-Jones to the exponential-6 potential of H2. The MWDA gives too low results for the repulsive Lennard-Jones potential. At low temperatures, it fails for the full LJ potential while at high temperatures it is in good agreement. Including the attraction as a mean-field correction gives good results also for low temperatures. The MWDA results are too high for the exponential-6 potentials. The MELA fails completely for the LJT potential and the hydrogen exponential-6 potential, since it does not give a stable solid phase.
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    Application of the reduced-equations-of-motion formalism to the scattering of molecular NO from a LiF (001) surface at nonzero temperature (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5288-5299 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reduced-equations-of-motion (REOM) model for gas–solid energy transfer in molecule–surface collisions [D. J. Diestler and M. E. Riley, J. Chem. Phys. 89, 4137 (1989)] is extended to include effects of nonzero surface temperature by combining the REOM damping theory with a stochastic partial velocity reset (PVR) algorithm [M. E. Riley, M. E. Coltrin, and D. J. Diestler, J. Chem. Phys. 88, 5934 (1988)]. The REOM/PVR procedure, which involves integration of the gas–molecule EOM above the frozen solid with stochastic resetting of the molecule's velocities, is tested by comparing results for NO+LiF(001) scattering with those of a previous stochastic-trajectory study of this system [R. R. Lucchese and J. C. Tully, J. Chem. Phys. 80, 3451 (1984)]. The REOM/PVR results reproduce the trends in the stochastic-trajectory results very well for translational and rotational energy transfer as a function of the various system parameters. However, it is found that the coupling of the vibrational mode, whose frequency is greater than that of the Debye frequency of the solid, is not treated accurately by the REOM theory, which is based on adiabatic approximation to the solid's response function.
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    A beam scattering study of the dynamics of CH+4(CH4,CH3)CH+5 reaction in the eV collision energy range: Three competing mechanisms of CH+5 formation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4916-4921 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A crossed-beam study of the reaction CH+4(CH4,CH3)CH+5 was carried out in the collision energy range 0.6–2.3 eV. Three distinct patterns were observed, which may be interpreted in terms of three competing mechanisms for CH+5 formation: proton stripping, H-atom pickup, and intermediate complex decomposition. The existence of a C2H+8 intermediate, stable towards dissociation, is suggested by the results. The relative weights of the three mechanisms were estimated as a function of collision energy.
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    Rotationally and translationally resolved hot atom collisional excitation of the CO2 Fermi mixed bend/stretch vibrational levels by time-dependent diode laser spectroscopy (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4922-4937 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High resolution IR diode laser absorption spectroscopy is employed to monitor the nascent rotational population distributions and transient linewidths in the Fermi mixed symmetric stretch/overtone bend vibrational mode of CO2 (1000/0200) following collisions with translationally hot hydrogen and deuterium atoms, produced from the 193 nm excimer laser photolysis of H2S or D2S. The nascent 1000/0200 rotational distribution produced by H* atom collisional excitation peaks at J∼26 and is well fit by a 747 K Boltzmann distribution. The transient linewidths are 1.5–3 times the ambient, room temperature CO2 Doppler linewidths, are∼0.002 cm−1 larger for D* atom collisions than H* atom collisions, and increase with increasing rotational quantum number. The experimentally determined relative cross sections for H* atom collisional excitation of CO2 vibrational states are as follows: 0001 antisymmetric stretch: 1000 Fermi mixed upper level: 0200 Fermi mixed lower level: 0220 bend≈1.0 : 0.6 : 0.6 : 0.4. The absolute cross section for inelastic collisional scattering of CO2 by H* atoms into 1000 J=38 is (1.4±0.8)×10−2 A(ring)2, and the total excitation cross section for the 1000 vibrational state is 0.37±0.21 A(ring)2. A statistical model and a simple quantized Landau–Teller model are unable to explain qualitatively the observed data; however, a breathing ellipsoid model, coupled with an IOS quantum scaling relation, reproduces the major features in the experimental data for both the 1000/0200 and 0001 states. The differences in the experimental data for distinct vibrational motions can be attributed to hot atoms sampling different regions of the potential surface.
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    On the performance of atomic natural orbital basis sets: A full configuration interaction study (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4982-4985 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The performance of atomic natural orbital (ANO) basis sets has been studied by comparing self-consistant field (SCF) and full configuration interaction (CI) results obtained for the first row atoms and hydrides. The ANO results have been compared with those obtained using a segmented basis set containing the same number of contracted basis functions. The total energies obtained with the ANO basis sets are always lower than the one obtained by using the segmented one. However, for the hydrides, differential electronic correlation energy obtained with the ANO basis set may be smaller than the one recovered with the segmented set. We relate this poorer differential correlation energy for the ANO basis set to the fact that only one contracted d function is used for the ANO and segmented basis sets.
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    The prediction of spectroscopic properties from quartic correlated force fields: HCCF, HFCO, SiH+3 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4965-4981 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Knowledge of a force field expanded through quartic displacements, together with a dipole field expanded through cubic displacements, yields all the harmonic and anharmonic molecular properties of interest to infrared spectroscopists. Such force fields may also explain much of the mechanism behind intramolecular vibrational energy redistribution. The ab initio quantum chemist can now calculate these fields, either at the self-consistent field level or with the inclusion of electron correlation effects. For accurate predictions, it is important to include electron correlations effects for at least the quadratic part of the force fields. Here we report studies using the second-order Møller–Plesset method for the full quartic fields. We examine the effects of using large basis sets. The quadratic force constants are calculated analytically; cubic and quartic constants are calculated using central differences of second derivatives in reduced normal coordinates. Three molecules are studied. HCCF, for which a large quantity of experimental data has been recently analyzed by Holland, Newnham, and Mills. The calculations are sufficiently accurate that errors in the experimental assignments became apparent. HFCO, where the theoretical anharmonic constants are helpful in understanding the highly excited vibrational states probed by Moore and co-workers. SiH+3, whose high resolution absorption spectra has just recently been detected by Davies and co-workers. The conclusions are that this straightforward way of calculating spectroscopic properties is an extremely valuable tool for the understanding of spectroscopy.
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    The potential energy and dipole moment surfaces of NF2 and O−3 by complete active space self-consistent field (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5020-5028 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Three-dimensional potential energy and dipole moment surfaces have been calculated for the 25 electron radicals NF2 and O−3 in their 2B1 ground electronic states by the complete active space self-consistent field (CASSCF) method with basis sets of 87 (NF2) and 99 (O−3) contracted Gaussian-type orbitals. Spectroscopic constants have been calculated from the analytical potential energy functions for each species, and the results for NF2 are compared to the available experimental data. Predictions of the rotational and rotational–vibrational spectra of O−3 have been made by comparison to the NF2 results. Vibrational band origins have been calculated by perturbation theory and also variationally in a basis of distributed Gaussian functions. Rotationless dipole moment matrix elements and vibrational band intensities have been determined from the CASSCF dipole moment functions. The fundamental vibrational band origins and intensities of O−3 are predicted to be 979 cm−1/0.87 cm−2 atm−1 (ν1), 565 cm−1/17.8 cm−2 atm−1 (ν2), and 739 cm−1/2620 cm−2 atm−1 (ν3). Smaller basis set calculations of the first three excited electronic states of O−3 have also been carried out, and the results are compared to previous photodissociation experiments.
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    The vibrational spectra of molecular ions isolated in solid neon. III. N+4 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3856-3862 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: When a Ne:N2=100 or 200 mixture is codeposited at 5 K with a beam of neon atoms excited by a microwave discharge, a weak to moderately intense infrared absorption appears at 2237.6 cm−1 which is assigned to the N+4 molecular ion. The analysis of the infrared spectra of the nitrogen-15 substituted species of N+4 supports the conclusion from earlier ab initio calculations and electron spin resonance observations that N+4 has a linear, centrosymmetric ground-state structure. For the N+4 species with noncentrosymmetric isotopic substitution, the in-phase end-atom stretching fundamental becomes infrared active and has also been observed. Although the anion responsible for overall charge neutrality of the deposit has not been definitively identified, secondary photolysis studies provide some information regarding its properties.
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    Determination of the transition dipole moment μi→b(R) in H2 from the measurement of vibrational wave functions (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3887-3890 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this work we present a theoretical and experimental study of the i 3Π−g→b 3∑+u transition dipole moment in molecular hydrogen. By means of translational spectroscopy the functional dependence on internuclear distance of the transition dipole moment was directly probed. Selective excitation of the i 3Π−g, N=1 level, being unaffected by rotational couplings with other 3d singly excited states, allows for straightforward comparison with ab initio calculations. Excellent agreement is found.
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    Multidimensional variational Gaussian wave packet dynamics with application to photodissociation spectroscopy (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3919-3930 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The McLachlan variational principle for the time-dependent Schrödinger equation is utilized in conjunction with extant localized Guassian wave packet technology to deduce equations of motion for general multidimensional Gaussians. These equations of motion are characterized by the same simplicity as the local quadratic expansion results of Heller [J. Chem. Phys. 62, 1544 (1975)]. However, the resultant variational wave packet evolution is shown to be an improvement over its local quadratic analog as a tool for computing certain photodissociation spectra. Numerical examples drawn from the Beswick–Jortner model of ICN photodissociation [Chem. Phys. 24, 1 (1977)] are presented.
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    Polarized emission spectroscopy of photodissociating nitromethane at 200 and 218 nm (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3958-3969 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the polarized emission spectra from photodissociating nitromethane excited at 200 and 218 nm. At both excitation wavelengths, the emission spectra show a strong progression in the NO2 symmetric stretch; at 200 nm a weak progression in the NO2 symmetric stretch in combination with one quantum in the C–N stretch also contributes to the spectra. We measure the angular distribution of emitted photons in the strong emission features from the relative intensity ratio between photons detected perpendicular to versus along the direction of the electric vector of the excitation laser. We find the anisotropy is substantially reduced from the 2:1 ratio expected for the pure CH3NO2 X(1A1)→1B2(ππ*)→X(1A1) transition with no rotation of the molecular frame. The intensity ratios for the features in the NO2 symmetric stretching progression lie near 1.5 to 1.6 for 200 nm excitation and 1.7 for 218 nm excitation. The analysis of the photon angular distribution measurements and consideration of the absorption spectrum indicate that the timescale of the dissociation is too fast for molecular rotation to contribute significantly to the observed reduction in anisotropy. The detailed analysis of our results in conjunction with electron correlation arguments and previous work on the absorption spectroscopy and final products' velocities results in a model which includes two dissociation pathways for nitromethane, an electronic predissociation pathway and a vibrational predissociation pathway along the 1B2(ππ*) surface. Our analysis suggests a reassignment of the minor dissociation channel, first evidenced in photofragment velocity analysis experiments which detected a pathway producing slow CH3 fragments, to the near threshold dissociation channel CH3 + NO2(2 2B2).
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    Quenching of O(1S) studied by a pulsed vacuum-ultraviolet-resonance lamp (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3978-3982 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rate constants for the quenching of metastable O(1S) by a number of collision partners, in particular hydrocarbons, were determined at room temperature. Care was taken to obtain data with high precision and to disturb the system as little as possible by the pulsed vuv photolytic generation of O(1S). Comparison with data for the other O atom states and isoelectronic NH states reveals little similarity.
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    Molecular orientation control for ||JKMJ〉 —selected CH3I via influence of electric field on nuclear hyperfine coupling (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4024-4032 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the polarized laser photofragment photoionization technique, measurements have been made of the degree of orientation of CH3I in ||JKMJ〉=||111〉 and ||222〉 parent rotational states under the influence of homogeneous electric fields (E) from 0 to 1.0 kV cm−1. From a series of experiments on hexapole-oriented molecules in weak fields, it has been found that the original degree of orientation of the symmetric-top molecules can be recovered after they pass through a homogeneous weak-field region provided that the field strength therein exceeds some minimum value (E(approximately-greater-than)0.3 V cm−1), sufficient to maintain an orientation axis. Even though the ||JKMJ〉 parent states have "relaxed'' via hyperfine coupling to an ensemble of ||FJKIMF〉 states, when the molecules later pass into a strong E field, the ensemble is restored to its original degree of orientation. Another set of experiments in moderate to strong fields provided "saturation curves,'' i.e., the dependence of the degree of orientation upon E. The results show that quite high field strengths (〉0.5 kV cm−1) are required to ensure total decoupling of J from I and thus recovery of the full orientation. From these experiments it is now clear that one can control the degree of molecular orientation by placing the state-selected molecules in a know E field.
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    Conformational instability of the lowest triplet state of the benzene nucleus. I. The unsubstituted molecule (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3733-3745 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Experiments on benzene have established that its lowest triplet state (3B1u) is conformationally unstable owing to vibronic coupling with the next higher state (3E1u). This instability was found to be critically dependent on the influence of a crystal field. An analogous vibronic coupling is to be expected in the singlet manifold, but here no direct evidence is available for a conformational instability. The distortion behavior of benzene is of importance for the interpretation of its photophysical and photochemical properties. We have therefore determined the potential-energy surfaces of the 1,3B1u and 1,3E1u states along the two-dimensional distortion coordinate S8(ρ,cursive-phi) using ab initio multireference single and double excitation-configuration-interaction calculations. The results show that for both B1u states the hexagonal conformation is unstable and lies 800 cm−1 above a wide, virtually cylindrical trough. A calculation of the vibrational spacing in the 3B1u state yields good agreement with the experimentally observed frequency. The calculation of intensities in the absorption and emission spectrum for this state qualitatively agrees with the experiment. An estimate is made of the interaction of the excited molecule with neighboring molecules in a crystal, which indicates that the crystal-field induced energy variations in the trough should be of the order of 10 cm−1. Combination of our calculations with experimental data shows that the vibronic coupling in the B1u states of benzene should not be looked upon as a static coupling in which the molecule is permanently distorted to one conformation but as a dynamic one in which the molecule makes excursions over the entire potential-energy surface.
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    Corrected effective medium method. V. Simplifications for molecular dynamics and Monte Carlo simulations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4413-4426 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present the conceptual and formal simplifications of the recently developed corrected effective medium (CEM) theory that enable this theory to be used directly in molecular dynamics (MD) and Monte Carlo (MC) simulations of large systems, hence the acronym MD/MC-CEM. The essential idea involves adjustment of the CEM embedding functions to include approximately the original explicit correction for kinetic-exchange-correlation energy differences between the real system and the many atom–jellium systems used as the zeroth order model. Examples of this construction are provided for the Ni, Pd, Ar, and H/Pd(111) systems. Finally, a few brief applications of this method to large systems are provided. These include relaxation of metal surfaces, structure of pure Ni and mixed NiCu clusters, sticking of Cu on Cu(100), and the scattering of Ar from H covered Pd(111).
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    Site–site correlations in short chain fluids (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4453-4461 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular and intermolecular site–site correlations in short chain fluids are obtained via Monte Carlo simulation for volume fractions ranging between 0.05 and 0.35. The chains are modeled as pearl necklaces of freely jointed hard spheres; chains composed of 4 and 8 beads are studied. The intramolecular distribution between a pair of beads separated by a fixed number of segments along the chain is found to be remarkably independent of the position of the pair along the chain. At low densities the intermolecular site–site pair distribution function at contact is found to be much less than one due to the "correlation hole'' effect. The contact value increases as the density is increased, and decreases as the chain length is increased. We use the intramolecular correlations measured to obtain polymer reference interaction site model predictions for the intermolecular site–site distribution function. We find that the theory accurately reproduces the local structure of the fluid, but significantly overestimates the contact value of the distribution function, especially at low densities. A comparison of freely jointed chain results with simulations of chains with fixed bond angles and torsional rotations treated in the rotational isomeric state approximation shows that the correlation hole is more pronounced in freely jointed chains. We test a superposition approximation used to evaluate the three body term in the pressure equation for chain molecules. We find that the three-body term is sizeable, and that the superposition approximation significantly underestimates the three-body contribution.
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    The B←X electronic spectrum of N2+–He (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4477-4478 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic spectrum of N2+–He has been measured in the region corresponding to the N2+ B 2Σ+u←X 2Σ+g origin transition. The spectrum was recorded by photoexciting a mass selected beam of N2+–He ions and detecting N2+ fragments. A likely process for the fragmentation involves fluorescence to a vibrationally excited level of the ground electronic state followed by vibrational predissociation. The observed spectrum exhibits well resolved discrete structure and bears a remarkable resemblance to a cold N2+ spectrum suggesting that the potential between the N2+ ion and helium atom in both the X and B electronic states, has at most only a small barrier to internal rotation. Measurement of the shift of N2+–He transitions with respect to the corresponding N2+ lines indicates that the binding energy of the helium atom to the N2+ ion is almost the same in both the B and X electronic states.
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    Analysis of the absorption spectrum of the NV1 transition of cyclopentadiene (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3850-3855 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultraviolet absorption spectra of cyclopentadiene and cyclopentadiene-d6 were measured in the NV1 transition region. The spectra were analyzed to determine the excited state properties. The prominent vibrational progression was identified as a progression in the (predominantly) a1 C(Double Bond)C stretching vibration. A second, unidentified, vibrational interval was observed in the cyclopentadiene spectrum and inferred to exist in the spectrum of cyclopentadiene-d6. The excited state potential surface was deduced to be displaced 0.2 A(ring) from the lower, predominantly along the C(Double Bond)C normal coordinate. The excited state lifetime was determined to be 37 fs at the origin and decrease at a rate inversely related to the excited state vibrational quantum number.
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    URL: http://dx.doi.org/10.1063/1.458770
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    Double-Rydberg anions: Ground-state electronic and geometric stabilities (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3874-3880 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Each member of the class of Double-Rydberg (DR) molecular anions consists of an underlying closed-shell cation core around which a pair of highly correlated electrons move in diffuse orbitals. We have examined the geometric and electronic stabilities of the ground states of candidate DR anions resulting from the following cation cores: H+3 , NeH+, FH+2 , H3O+, NH+4 , and CH+5 . Near the equilibrium geometry of the cation, all of the DR anions, except H−3 , are electronically stable with respect to the corresponding Rydberg radicals. Results of our geometry optimizations indicate, however, that only NH−4 and H3O− are locally geometrically stable; the other DR anions undergo fragmentation. Vertical ionization potentials for the Td isomer of NH−4 and the C3v isomer of H3O− are found to be 0.45 and 0.46 eV, respectively.
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    URL: http://dx.doi.org/10.1063/1.458773
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  • 83
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    Continuous wave multi-quantum electron paramagnetic resonance spectroscopy (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3891-3898 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equation of Anderson [Phys. Rev. 102, 151 (1956)] (ω2−ω1)2=(γH0−ω1)2+γ2H21, which describes resonance conditions if relaxation times are long and irradiation at two frequencies is applied to a spin system, has been studied experimentally in the context of continuous wave electron paramagnetic resonance (EPR) spectroscopy. Here ω2 and ω1 are the frequencies of two incident microwave levels, one of which is much stronger than the other and is of amplitude H1. γH0 is the resonant condition if just one frequency is applied. Magnetization at either ω1 or ω2 has been observed as a function of sweep of the static magnetic field, sweep of ω2 and also sweep of the amplitude H1. Observation of magnetization at frequency ω1 corresponding to the strong microwave field H1 replicates the rotary saturation experiment of Redfield [Phys. Rev. 98, 1787 (1955)]. Multi-quantum effects are studied with the two frequencies well separated and also when they lie within the width of a single homogeneous line. In addition, data are shown when both microwave amplitudes are similar and the Anderson equation is no longer correct. The thrust of the work is not only to study the spin physics, but also to develop a basis for our development of rotary resonance as an alternative to field modulation in EPR spectroscopy [J. Chem. Soc. Faraday Trans. 1 85, 3901 (1989)].
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    URL: http://dx.doi.org/10.1063/1.458775
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  • 84
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    Double continuum fragmentation in the vibrational predissociation X⋅⋅⋅BC(v)⋅⋅⋅Y→BC(v'〈v)+X+Y of van der Waals complexes: A perturbative treatment (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4016-4023 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an approximate quantal model to study the double continuum problem arising in the complete fragmentation of X⋅⋅⋅BC(v)⋅⋅⋅Y van der Waals(vdW) complexes, where BC is a conventional diatomic molecule vibrationally excited and X and Y are rare gas atoms, through vibrational predissociation (VP). Assuming a near equilibrium geometry of the complex and using an adiabatic approximation for describing the oscillation in the angle formed by the BC⋅⋅⋅X and BC⋅⋅⋅Y weak bonds, the rates for complete fragmentation are expressed in the frame of Fermi's "Golden Rule''. Double continuum wave functions may be obtained by a perturbative treatment that allows one to take properly into account the symmetry of the problem in the particular and very frequent case X≡Y.
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    URL: http://dx.doi.org/10.1063/1.458786
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  • 85
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    Kinetic energy released in positive ion pair formation from inner-shell excited oxygen molecule (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4073-4076 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Total kinetic energy released in dissociation from inner-shell excited states of O2 induced by soft x-ray irradiation has been measured using a time-of-flight mass spectrometer operated in a photoion–photoion coincidence mode. The distribution of the total kinetic energy released in the dissociation into O++O+ at a photon energy above the 1s ionization threshold shows three peaks at about 6, 11, and 15 eV and that at an energy of the 1s→2pπg transition has a broad maximum at 11 eV. The total kinetic energy distribution of O++O2+ shows a broad maximum at about 20 eV in all the photon energies. The dissociation pathways are discussed in detail using the kinetic energy distributions obtained.
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    URL: http://dx.doi.org/10.1063/1.458739
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  • 86
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    Picosecond energy transfer of vibrationally hot molecules in solution: Experimental studies and theoretical analysis (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4094-4101 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The cooling of vibrationally hot azulene is studied in different solvents by picosecond spectroscopy. Excitation to the electronic S1 state generates molecules with a vibrationally hot ground state by rapid internal conversion. The subsequent cooling is monitored by the temperature-dependent change of the S0–S1 absorption edge and occurs via interaction with the solvent on a time scale of several tens of picoseconds. A theoretical model of intermolecular energy transfer in the liquid phase is developed. The vibrational excess energy of azulene is transferred to the solvent molecules by isolated binary collisions, where the multimode vibrational system of the molecules is considered explicitly. The dissipation of energy within the solvent is simulated by the macroscopic conduction of heat. The temporal development of the vibrational temperature of the azulene molecules and the concommitant changes of absorption are calculated taking into account the properties of the specific solvent. The results of the theory show quantitative agreement with our data.
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    URL: http://dx.doi.org/10.1063/1.458741
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  • 87
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    The ligand field spectrum of the hexafluorochromate (III) anion: An ab initio study including correlation effects (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4154-4163 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and ligand field excited states of CrF3−6 have been studied, using different Gaussian basis sets of atomic natural orbitals. Each state was first optimized separately in a complete active space self-consistent field (CASSCF) calculation, including three active electrons in the 2t2g and 4eg shells. Further correlation was then added by using either a singles and doubles configuration interaction approach (SDCI) or by the average coupled pair functional method (ACPF) with the CASSCF configuration space as the reference space. Thereby the number of correlated electrons was raised up to 15. It is shown that the quartet–quartet transitions, corresponding to a 2t2g→4eg excitation, are described already very accurately at the CASSCF level. Further improvement of the 4A2g→4T2g transition was obtained by extending the CI treatment so as to include the F 2p electrons from the 1t2g,3eg, and finally also from the 6a1g shell. For the intraconfigurational t32g quartet–doublet transitions on the other hand, the best results were obtained by an 11 electron CI treatment, including the Cr(III) 3s and 3p electrons.
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    URL: http://dx.doi.org/10.1063/1.458748
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  • 88
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    The permanent electric dipole moment of CaOH (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4179-4186 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical calculations yield electric dipole moments ( μ) of 0.98, 0.49, and 0.11 Debye at the computed equilibrium geometries of the X 2Σ+, A 2Π, and B 2Σ+ states of CaOH. Thus the pure rotational spectra in these electronic states are expected to be weaker than the corresponding alkaline monohalides. This is consistent with our failure to observe the pure rotational spectrum of the X 2Σ+ state by microwave-optical double resonance spectroscopy, and is in agreement with our experimental determination of μ〈1 D for the A 2Π3/2 state. However, the vibrational–rotational spectrum is quite strong, since dμ/dr is quite large in these ionic molecules. Using the free-ion polarizabilities, the modified Rittner model predicts a μ value of about −8 D for the X 2Σ+ state, which is in error by about 9 D. The theoretical values for total dipole moment, as well as contributions from Ca+ and OH−, can be reproduced using effective polarizabilities. However, this analysis shows that the effective polarizability of OH− is a much smaller fraction of the free-ion value than for Ca+, because Pauli repulsion restricts the OH− polarization toward the Ca+. Some account of this effect seems to be a prerequisite for the successful application of either Rittner or ligand field models to ionic systems.
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  • 89
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    Electronic structure of vinylthio radical (CH2CHS): A theoretical study (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4223-4229 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A predictive ab initio MO calculation was performed for the thus far undetected sulfur analog of the vinoxy radical. Multiconfigurational self-consistent field (MCSCF) and symmetry adapted cluster expansion–configuration interaction (SAC–CI) wave functions were used to describe the electronic structure of the radical. The symmetry species of the ground state was found to be 2A‘ as in the case of the vinoxy radical. The crucial difference between the two radicals is that the thio-radical is more or less characterized as a sulfur-centered radical, CH2=CH–S⋅ whereas the oxy-radical is regarded as a carbon centered ⋅CH2–CH=O in their ground state. Excitation energies, normal modes, Franck–Condon factors, hyperfine coupling parameters, and electron affinity were predicted.
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    URL: http://dx.doi.org/10.1063/1.458754
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  • 90
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    Refractive index of polarizable fluids: Numerical results from the mean spherical approximation solution (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4272-4280 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The numerical solution of the MSA (mean spherical approximation) for a fluid of polarizable particles interacting via the electromagnetic dipole radiation field is presented. The effects of this interaction are studied for molecules with polarizabilities both independent and dependent on frequency. Imaginary and real parts of the refractive index are evaluated, and their behaviors in connection with the absorption bands are determined. Finally, time-correlation and response functions have been evaluated.
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    URL: http://dx.doi.org/10.1063/1.458760
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  • 91
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    Fourth adiabatic ensemble (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4296-4298 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The statistical mechanics of a new adiabatic ensemble, which allows volume and particle fluctuations, is presented. Characteristic fluctuation formulas and suitable response functions are derived.
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    URL: http://dx.doi.org/10.1063/1.458710
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  • 92
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    Calculation of the potential of mean force using molecular dynamics with linear constraints: An application to a conformational transition in a solvated dipeptide (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4312-4321 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An algorithm is proposed for the calculation of mean force potentials using molecular dynamics with linear constraints. The new algorithm is based on free energy perturbation and is especially useful when the analytical form of the reaction coordinate is not known (e.g., for a numerically evaluated steepest descent path). It employs only Cartesian coordinates which makes it easy to use for a variety of systems. In particular, it is useful when the reaction coordinate cannot be represented by a single local coordinate or by a combination of a small number of local (internal) coordinates. The linear constraints are solved explicitly and the computational effort required is similar to that of dynamics with no constraints. The potential of mean force for a conformational transition of a solvated valine dipeptide is calculated. In aqueous solution the barriers for the conformational transitions are reduced with respect to the barriers in vacuo, the helix configuration becomes stabilized.
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    URL: http://dx.doi.org/10.1063/1.458713
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  • 93
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    Effect of pressure on conformational equilibria of liquid 1-chloropropane and 1-bromopropane (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4345-4348 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effects of pressure on conformational equilibria of liquid 1-chloropropane and 1-bromopropane has been measured up to 2.5 kbar by Raman spectroscopy. Pressure effects on relative integrated intensities of the carbon–halogen stretching vibration of the two conformers show that both gauche conformers increase when compressed. The volume changes of the trans to the gauche are −1.1±0.2 and −1.2±0.2 cm3 /mol for 1-chloro- and 1-bromopropanes at 20 °C, respectively. The volume changes are mainly due to the solvation volumes as shown from calculation of the van der Waals volumes. The solvation volume cannot be accounted for by the quadrupole-induced electrostatic interactions in liquids. The solvation volume arises from local intermolecular interactions.
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    URL: http://dx.doi.org/10.1063/1.458717
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  • 94
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    Elementary processes in the scattering of NO molecules from a diamond surface (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4368-4376 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report on the scattering of supersonic NO molecules from a diamond (111) surface. Using state selective laser techniques combined with mass selective quadrupole measurements, we observe two novel features in the molecule–surface interaction: first, a scattering behavior which is dependent on the direction of the incoming molecules; and second, surface residence times that are unusually long, but temperature independent. The former observation can be traced back to the topography of the unreconstructed diamond (111) surface, the latter one can be understood when a temporary function of a metastable surface–molecule complex is assumed.
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    URL: http://dx.doi.org/10.1063/1.459672
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  • 95
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    Diffusion coefficients and effective viscosity of suspensions of sticky hard spheres with hydrodynamic interactions (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4427-4432 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the short-time and long-time diffusion coefficients and the high-frequency and zero-frequency effective viscosity of a dilute suspension of sticky hard spheres with hydrodynamic interactions. Due to the singular nature of the hydrodynamic interactions near touching the transport coefficients are strongly affected by the short-range attractive potential. This suggests that the transport coefficients may be used as a test of the interaction potential.
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    URL: http://dx.doi.org/10.1063/1.459688
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  • 96
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    Kinetics of electrically and chemically induced swelling in polyelectrolyte gels (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4462-4472 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Controlled swelling and shrinking of polyelectrolyte gels is useful for regulating the transport of solutes into, out of, and through these materials. A macroscopic continuum model is presented to predict the kinetics of swelling in polyelectrolyte gel membranes induced by augmentation of electrostatic swelling forces arising from membrane fixed charge groups. The model accounts for ionic transport within the membrane, electrodiffusion phenomena, dissociation of membrane charge groups, intramembrane fluid flow, and mechanical deformation of the membrane matrix. Model predictions are compared with measurements of chemically and electrically induced swelling and shrinking in crosslinked polymethacrylic acid (PMAA) membranes. Large, reversible changes in PMAA membrane hydration were observed after changing the bath pH or by applying an electric field to modify the intramembrane ionic environment and fixed charge density. A relatively slow swelling process and more rapid shrinking for both chemical and electrical modulation of the intramembrane pH are observed. The model indicates that retardation of membrane swelling is dominated by diffusion-limited reaction of H+ ions with membrane charge groups, and that the more rapid shrinking is limited primarily by mechanical processes.
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    URL: http://dx.doi.org/10.1063/1.458729
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    The vibrational quantum number dependence of the collisional lifetime in nitric oxide self-relaxation up to v‘=25 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4483-4484 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458685
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  • 98
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    Some results for hard spheres at a hard wall using the inhomogeneous Percus–Yevick equation and a modified Lovett–Mou–Buff–Wertheim equation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4489-4490 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Some results are reported for the contact value of the density profile of hard spheres near a hard wall using the inhomogeneous Percus–Yevick equation for pair correlation functions and the Lovett–Mou–Buff–Wertheim (LMBW) equation to relate the pair functions to the singlet density profile. It is found that improved results are obtained if the LMBW is modified as suggested by Quintana et al.
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  • 99
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    Erratum: Interference effects in surface enhanced Raman scattering by thin adsorbed layers [J. Chem. Phys. 92, 4600 (1990)] (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4492-4492 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459726
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  • 100
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    High resolution Raman spectroscopy in the α and β crystalline phases of N2 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3005-3011 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The line shape of the Eg libron at 32 cm−1 and of the two components (Ag and Tg) of the vibron at 2330 cm−1 of solid α-N2 at normal pressure were measured as a function of temperature between 5 and 35.6 K by means of high-resolution Raman spectroscopy with a limiting spectral resolution of 0.006 cm−1. A suitable deconvolution procedure provides the respective weights of Lorentzian and Gaussian contributions to the true phonon line shapes. The measured residual bandwidth of the Eg libron at low temperature is noticeably narrower than that previously quoted in the literature (0.16 cm−1 instead of 0.8 cm−1). At high temperature it is shown that the libron relaxation mechanisms are mainly governed by four-phonon processes. The temperature dependence of the vibron dephasing mechanisms are discussed in terms of contributions arising from depopulation, elastic and inelastic quartic processes as well as from disorder. The influence of the low energy optical phonons (ω(approximately-equal-to)32 cm−1) in the vibron relaxation mechanisms is emphasized. It is shown that between 25 K and the α–β transition temperature at Tc=35.6 K, the temperature dependent vibron linewidths are driven by inelastic processes. Finally some results on the frequency and the bandwidth of the ν1 mode in the β phase are reported.
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    URL: http://dx.doi.org/10.1063/1.458887
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