ALBERT

Notizen: We report striking observations of coherent caging of iodine, above the B state dissociation threshold, by single collisions with rare gas atoms at room-temperature. Despite the random nature of the solute–solvent interaction, the caged population retains coherence of the initially prepared unbound wave packet. We discuss some new concepts regarding dynamical coherent caging and the one-atom cage effect.© 1997 American Institute of Physics.

Notizen: The concentration dependence of PCARS intensity has been measured for the 992 cm−1 band of benzene in five different solvents and compared with that of Rayleigh scattering intensity. The temperature dependence of PCARS and Rayleigh scattering has also been studied for neat benzene. The concentration dependence of PCARS and Rayleigh scattering shows a similar correlation to the refractive index difference between benzene and the solvents. It is shown that PCARS cannot be considered as the Rayleigh scattering of the (phase mismatched) CARS radiation, by comparing the concentration and temperature dependence of their intensities. The fluctuation of the third-order nonlinear susceptibility itself plays an important role in PCARS. This means that PCARS may bear unique information on the microscopic properties of liquids that Rayleigh scattering can not provide. © 1997 American Institute of Physics.

Notizen: We present a method for obtaining the thermal rate constant directly (i.e., without first solving the state-to-state reactive scattering problem) from the time integral of the flux-flux autocorrelation function, Cff(t). The quantum mechanical trace involved in calculating Cff(t) is efficiently evaluated by taking advantage of the low rank of the Boltzmannized flux operator. The time propagation is carried out with a Hamiltonian which includes imaginary absorbing potentials in the reactant and product exit channels. These potentials eliminate reflection from the edge of the finite basis and ensure that Cff(t) goes to zero at long times. In addition, the basis can then be contracted to represent a smaller area around the interaction region. We present results of this method applied to the O+HCl reaction using the J-shifting and helicity conserving approximations to include nonzero total angular momentum. The calculated rate constants are compared to experimental and previous theoretical results. Finally, the effect of deuteration (the O+DCl reaction) on the rate constant is examined. © 1997 American Institute of Physics.

Notizen: Formulas governing fixed orbital hardnesses and their relation to the hardness kernel are derived. It is shown how the orbital hardness matrix and its inverse matrix, the orbital softness matrix, may thus be directly calculated, and then the total chemical hardness, softness, and electronegativity of a molecular species. These quantities are calculated for the molecule HCN, using Dirac exchange and von Barth–Hedin correlation in the local density form of Kohn–Sham theory. The result complies with the frontier orbital theory. As quantitative indicators of orbital reactivity, the frontier orbital softness and Fukui indices generally have larger values than inner electron orbitals. The relation of orbital hardness matrix elements to the two-electron orbital integrals in a typical molecular orbital calculation is discussed, and it is demonstrated that diagonalization of the orbital hardness matrix leads to orbitals more localized than conventional Kohn–Sham orbitals. © 1997 American Institute of Physics.

Notizen: We show that in the molecular dynamics simulation of systems where the molecules are subjected to holonomic constraints the isobaric–isothermal (NPT) techniques can be applied by scaling in space the atomic coordinates rather than the molecular center of mass, as has been customary until now. The correct constrained distribution function is obtained by applying the original Andersen transformation or equivalently the Hoover-type NPT equations of motion. The latter set of equations are shown to be derived from an underlying Lagrangian. The atomic scaling is preferred to the molecular one when simulating large molecules since molecular metastabilities are more easily overcome. However, also for systems composed of small molecules, the approach can be interesting since in this way cell rotations are automatically avoided. © 1997 American Institute of Physics.

Notizen: The CF+ and CF3+ fragment ion yield curves from C2F4 have been remeasured by photoionization mass spectrometry. Fits with appropriate model curves yield the appearance potentials AP0(CF3+C2F4)=13.721±0.005 eV and AP0(CF+/C2F4)=13.777±0.005 eV and an accurate difference in ionization potentials, IP(CF)−IP(CF3)=0.055±0.003 eV. With the existing photoelectron value IP(CF)=9.11±0.01 eV, this produces IP(CF3)=9.055±0.011 eV. The CF3+ fragments from CF3Cl, CF3Br, and CF3I have also been remeasured, and their ion yield curves fitted with model functions. The experimentally derived AP0(CF3+/CF3Cl)〈12.867±0.008 eV has been found to be only an upper limit. The Analogous CF33CF3+ fragment yield curves from CF3Br and CF3I produce AP0(CF3+/CF3Br)=12.095±0.005 eV and AP0(CF3+/CF3I)=11.384±0.005 eV, leading to D0(CF3−Br)=70.1±0.3 kcal/mol (70.8±0.3 kcal/mol at 298 K) and D0(CF3−I)=53.7±0.3 kcal/mol (54.3±0.3 kcal/mol at 298 K). Based on tabulated values for ΔHf&convolu;(CF3Br) and ΔHf&convolu;(CF3I), which appear to be inconsistent by ∼1 kcal/mol, a compromise value of ΔH298 f&convolu;(CF3)=−111.4±0.9 kcal/mol (−110.7±0.9 kcal/mol at 0 K) is selected, resulting in ΔH 298f&convolu;(CF3+)=97.4±0.9 kcal/mol (98.1±0.9 kcal/mol at 0 K). Additionally, IP(CF4)≡AP0(CF3+/CF4)=14.67±0.04 eV can be inferred. From data on C2F4, ΔH298f&convolu;(CF)=62.5±1.1 kcal/mol (61.7±1.1 kcal/mol at 0 K) can be deduced. Many earlier literature values for appearance potentials of CF3+ from CF3X, leading to very low ΔHf&convolu;(CF3+) and/or IP(CF3) values, are demonstrated to be in error. © 1997 American Institute of Physics.

Notizen: The phase change possibilities in the surface phase formed in a binary liquid system (water 2,5dimethylpyridine) in contact with a solid (silica) have been examined on the basis of adsorption isotherms determined at ten different temperatures. The liquid mixture undergoes a liquid–liquid demixing but also, some 20° below the critical point, a solid phase separation. On the l.h.s. of a liquid–liquid coexistence curve 2,5dimethylpyridine, diluted in water, adsorbs step by step, and on the r.h.s. the relative adsorption is wave shaped. Both behaviors can be explained by a layering process, i.e., a solid–liquid surface demixing, which unexpectedly does not exclude a separate liquid–liquid surface demixing, which could constitute the prewetting process. This analysis is compared with known theories and its consistency is carefully checked on a thermodynamical basis. © 1997 American Institute of Physics.

Notizen: Recent controversy in the literature about the structure of water away from ambient conditions is manifested by significant differences between the site–site pair correlation functions of water as derived from neutron diffraction data and the same quantities obtained in computer simulations using an effective pairwise potential for the intermolecular interactions. One possible explanation of the discrepancies between computer models of water and neutron diffraction results near the critical point is that they arise from uncertainties in the inelasticity correction, which is particularly large for light water. To test out this idea, a new method of obtaining the pair correlations is described. This new analysis method is applied both to the earlier neutron data on non-ambient water and on recently reported data for a range of densities at 573 K. For this newer data the incident neutron spectrum is derived from an ambient water moderator instead of the liquid methane (100 K) moderator of previous work, and this has the effect of altering the shape of the inelasticity distortion to the diffraction data. It is found that the resulting site–site correlation functions from different experiments are now closely consistent with one another, indicating that the inelasticity correction has been successfully performed by this new analysis. It is also demonstrated that the new site–site correlation functions can be generated from three-dimensional distributions of water molecules. The previously reported discrepancies between simulation and neutron data are not as severe with this new analysis, but there remain systematic trends with density and temperature which apparently are not reproduced by computer simulation with empirical pair potentials. © 1997 American Institute of Physics.

Notizen: The kinetics of diatomic molecules adsorbing onto single-crystal surfaces is considered when this process can occur by a combination of dissociative adsorption and surface abstraction. The latter process adsorbs one atom on the surface and scatters the other from the surface. This combination of adsorption mechanisms controls the adsorption of fluorine on silicon surfaces [Y. L. Li et al., Phys. Rev. Lett. 74, 2603 (1995); J. A. Jensen et al., Science 267, 493 (1995)]. The effects of a variety of phenomena on the adsorption kinetics are explored, including surface diffusion, adlayer ordering due to adsorbate–lateral interactions, and the existence of weakly bound physiosorbed precursor states. When possible, exact expressions characterizing the adsorption kinetics are derived. Otherwise, approximate expressions are derived and compared with Monte Carlo simulations. © 1997 American Institute of Physics.

Notizen: The spatial distribution of N2 and NO desorbing from Rh(533), Rh(S)-[4(111)×(100)], was measured along the [65¯ 5¯] direction (along the steps) after adsorption of NO at 320 K. The N2 desorption at 450 K and 600 K showed a specific spatial distribution, that is, there was no peak either normal to the (111) terraces (θ=+14.4°) or normal to the (100) steps (θ=−40.3°), but a desorption maximum around θ=−15°. Integrated TPD spectra of N2 from the NO preadsorbed Rh(533) surface were very similar to those on Rh(111), but the spatial distribution of N2 from Rh(533) indicates desorption of N2 from the boundary between the (111) terraces and the vacant (100) steps. The spatial distribution of NO desorbing from the Rh(533) surface at 420 K obeyed cos θ, indicative of desorption normal to the crystal surface. Contrary to this, the spatial distribution of the desorption of NO molecules produced by the recombination reaction of adsorbed N and O desorbed at 550 K exhibits a peak intensity at θ=+15° (normal to the (111) terraces). © 1997 American Institute of Physics.

Notizen: Inner-shell excitation spectra of gaseous monocyanoethylene, trans-dicyanoethylene, and allylcyanide have been measured at C1s and N1s edges using 2 keV electron collisions in quasi-dipolar excitation conditions and the electron energy loss spectroscopy. The energy resolution has allowed the observation of transitions from different carbon sites. Ab initio calculations have been made to help the assignment of the experimental features. The spectra below the core-electron ionization limit have been interpreted in terms of transitions to the lowest-energy valence unoccupied molecular orbitals shown to be of π* type in each molecule. The effects of the conjugation between the multiple bonds on the spectra and on the splitting of the π* type molecular orbitals have been discussed. Strong electronic relaxation effects have been evidenced. © 1997 American Institute of Physics.

Notizen: In an earlier paper, a nonaffine network model of polymer melts was presented in which the rotation caused by shearing as well as the extension of the test strand are hindered by interactions with the network itself. In that work, it was shown that such a strand motion leads to qualitatively correct steady shear and elongational material properties, even though the strand disentanglement rate was constant and the strand force law was linear. These simplifications were accepted in order to emphasize the effects of the strand motion on material properties. In this paper, however, we show that these idealizations cause the model to fail in the start-up of shearing flow because no overshoot is seen in the shear stress growth function. To address this failure, the finitely extensible nonlinear elastic (FENE) network model is introduced in which the FENE force law replaces the Hookean force law used in the earlier finitely extensible network strand (FENS) model. Also, a nonlinear expression for the kinetics of strand disentanglement replaces the assumption of a constant rate of disentanglement. Material properties for the FENE network model are generated by stochastic simulation. The simulation results show that these modifications produce overshoot in the shear stress growth function and result in a more consistent description of finite network strand extensibility. © 1997 American Institute of Physics.

Notizen: Structural data on reverse micelles from the literature are analyzed from the adhesive spheres mixture model in the Percus–Yevick approximation. The solvent is modeled by a fluid of pseudoadhesive spheres with same compressibility and coordination number as a square well fluid. The attractive part of the direct intermicellar potential is taken proportional to the overlap volume of interpenetrating surface layers. The micelle–solvent coupling is characterized by a square or triangular well and a constraint on the packing fractions. A very good correlation is then found between the micelles stickiness parameters computed from the actual potentials via the equality of second virial coefficients and those determined from a fit to experiment. The change in effective adhesion observed with micelles of different surface composition and in the same solvent can then be associated to the change in the direct intermicellar interaction at nearly unchanged solvent–micelle interactions. The interpretation of the role of the overlap volume resulting from this multicomponent approach is contrasted with that based on approximate calculations of the potential of mean force. © 1997 American Institute of Physics.

Notizen: Recently a mode coupling theory for the dynamics of solutions and melts of entangled linear chain polymers has been developed. We report the extension of this approach to macromolecular architectures different from linear chains. Specifically, this work addresses recent experimental findings on melts of ring shaped polymers, small spherical micro-networks, and linear chains in two dimensions. The mechanical and dielectric response, diffusion, and molecular relaxation times of macromolecules modeled by fractal mass distributions are studied. The distribution is chosen to be Gaussian and then is uniquely determined from the experimentally measured scaling of macromolecular size (Rg) with degree of polymerization (N), i.e., Rg∝Nν. The exponent ν and the spatial dimension d determine the large N scaling of the transport coefficients and the exponents describing intermediate time anomalous diffusion. Within the theory, entanglement corrections to the single polymer Rouse dynamics are effective for ν〈2/d only. There, we find D∝N2dν−5 for the diffusion coefficient and that the ratio DτD/Rg2 is almost constant, where τD is the terminal relaxation time. Using independent input from equilibrium liquid state theories, the magnitude and scaling with macromolecular density and segment length of the dynamical properties is determined. It is also found that macromolecular interpenetration requires progressively higher densities and consequently entanglements become less effective with fractal dimension 1/ν approaching the spatial dimension. © 1997 American Institute of Physics.

Notizen: The results of molecular dynamics computer simulations of water laminae confined between two (9-3) Lennard-Jones walls are reported. The interaction potential is identical to the one used in a recent singlet integral equation study [M. J. Booth, D.-M. Duh, and A. D. J. Haymet, J. Chem. Phys. 101, 7925 (1994)]. In the simulation studies the internal pressure, system size, and the treatment of long-range Coulomb interactions has been varied. Large density oscillations similar to the integral equation results have not been observed. Simulated pair correlation functions are found to exhibit only little anisotropy near the interface. © 1997 American Institute of Physics.

Notizen: Single crystal adsorption calorimetry was applied to investigate the heats of adsorption of CO and oxygen and the reaction heats for the CO oxidation process on Pt{111} at room temperature. Both sticking probabilities and heats of adsorption for CO and oxygen are presented as a function of coverage. These results are used to interpret the subsequent measurements taken for the CO oxidation process on the same surface. The initial heats of adsorption of CO and oxygen on Pt{111} are 180±8 and 339±32 kJ/mol, respectively. In addition the pairwise lateral repulsive interaction between CO molecules in a ((square root of)3×(square root of)3)R30° ordered layer at θ=1/3 is found to be 4 kJ/mol. A detailed Monte Carlo modeling of the dissociative adsorption and sticking probability of oxygen on Pt{111} is performed. The initial rapid fall in heat is attributed to adsorption on defect sites, and subsequent adsorption on the planar {111} surface proceeds with a third neighbor interaction energy between the oxygen adatoms ω3∼22 kJ/mol. When gaseous CO reacts with preadsorbed oxygen adatoms, the CO2 produced has an excess energy of 16±8 kJ/mol. © 1997 American Institute of Physics.

Notizen: A new method is developed for study of photon- or electron-stimulated desorption from noninsulator surfaces. Concise and physically transparent, closed form expressions for the observables in a desorption experiment are given in terms of matrix elements of the interaction in stationary bases. The method is nonperturbative and spans the limits of weak to strong substrate–adsorbate interactions. We propose the possibility of relaxing several approximations frequently adopted in theoretical studies of desorption induced by electronic transitions. In particular, continuous decay of the transient electronic excitation produced by hot substrate carriers and the possibility of multiple electronic transitions follow naturally from the formulation and need not be introduced ad hoc. The description of time-delayed desorption, resulting from occurrence of predesorption resonances, entails neither conceptual nor computational difficulty and vibrational relaxation is consistently accounted for. The method is tested by application to an empirical model of the NH3/Cu photodesorption dynamics. © 1997 American Institute of Physics.

Notizen: The first eigenstate resolved, near the infrared spectrum of benzene in the region of the first C–H stretch overtone (6000 cm−1) has been obtained with an IR–IR double-resonance molecular beam optothermal spectrometer. Using a hierarchical tree analysis and level spacing statistics, we show that the intramolecular vibrational relaxation occurs nonergodically over at least seven different time scales ranging from 100 fs to 2 ns.© 1997 American Institute of Physics.

Notizen: Stimulated emission pumping (SEP) by two tunable, ns dye lasers selectively creates metastable V*(3d44s,a4DJ*) excited states near 8500 cm−1 in a fast flow reactor with 1.2 Torr He buffer gas at 300 K. Subsequent collisions with He equilibrate the spin-orbit levels J*=1/2–7/2 on a time scale of 1 μs but do not quench the 3d44s, a4D multiplet on a time scale of at least 50 μs. A third probe laser placed 5 mm downstream of the SEP lasers monitors the V* population at fixed reaction time t=40 μs. The exponential decay of V* vs calibrated hydrocarbon flow yields total removal rate constants that include both chemical reaction and electronic quenching of the V* excited state. Coupling of diffusion with reaction/quenching evidently does not cause the slight positive curvature observed in the kinetics plots. We demonstrate this by solving two simplified models of the reacting, diffusing, flowing gas for our experimental geometry, with SEP and probe laser beams transverse to the flow tube axis. Instead, the curvature is probably caused by the delayed, collisional cascade of a small population of higher excited states into the V* state. Ethylene removes the V* state on essentially every hard-spheres collision. The alkanes CH4, C2H6, and C3H8 remove V* on one in 75, one in 25, and one in 12 collisions. This is remarkably inefficient reaction/quenching, given that the low-spin 3d44s state is ideally configured for CH or CC bond insertion in alkanes. We interpret these new results and earlier work of Honma and co-workers on the sextet 3d44s excited state in terms of avoided intersections between repulsive and attractive diabatic potential surfaces (conserving electron spin and configuration). © 1997 American Institute of Physics.

Notizen: Computation of the Fock matrix is currently the limiting factor in the application of Hartree-Fock and hybrid Hartree-Fock/density functional theories to larger systems. Computation of the Fock matrix is dominated by calculation of the Coulomb and exchange matrices. With conventional Gaussian-based methods, computation of the Fock matrix typically scales as ∼N2.7, where N is the number of basis functions. A hierarchical multipole method is developed for fast computation of the Coulomb matrix. This method, together with a recently described approach to computing the Hartree-Fock exchange matrix of insulators [J. Chem. Phys. 105, 2726 (1900)], leads to a linear scaling algorithm for calculation of the Fock matrix. Linear scaling computation the Fock matrix is demonstrated for a sequence of water clusters at the restricted Hartree-Fock/3-21G level of theory, and corresponding accuracies in converged total energies are shown to be comparable with those obtained from standard quantum chemistry programs. Restricted Hartree-Fock/3-21G calculations on several proteins of current interest are documented, including endothelin, charybdotoxin, and the tetramerization monomer of P53. The P53 calculation, involving 698 atoms and 3836 basis functions, may be the largest Hartree-Fock calculation to date. The electrostatic potentials of charybdotoxin and the tetramerization monomer of P53 are visualized and the results are related to molecular function. © 1997 American Institute of Physics.

Notizen: One of the simplest organosulfur reactions, that between ground state carbon atoms, C(3Pj), and hydrogen sulfide, H2S(X1A1), was studied at an average collision energy of 21.0 kJ mol−1 using the crossed molecular beams technique. The product angular distribution and time-of-flight spectra of m/e=45 (HC32S) were monitored. Forward-convolution fitting of our data yields an almost isotropic center-of-mass angular flux-distribution, whereas the center-of-mass translational energy flux distribution peaks at about 50 kJ mol−1, indicating a tight exit transition state from the decomposing thiohydroxycarbene HCSH complex to the reaction products. The high energy cut-off of the translational energy flux distribution is consistent with the formation of the thioformyl radical HCS in its X2A electronic ground state. The first experimental verification of an existing thiohydroxycarbene intermediate and the rigorous assignment of the HCS radical product under single collision conditions explicitly suggest inclusion of the title reaction in chemical reaction networks of molecular clouds TMC-1 and OMC-1, the outflow of the carbon star IRC+10216, Shoemaker/Levy 9 impact-induced nonequilibrium sulfur chemistry in the Jovian atmosphere, as well as combustion of sulfur containing coal.© 1997 American Institute of Physics.

Notizen: Instantaneous normal mode (INM) theory was used to calculate absolute far-infrared absorption spectra of water, ammonia, and chloroform. Three procedures for weighting the INM density of states to yield absorption intensities were tested against spectra based on dipole time correlation functions generated from molecular dynamics (MD) simulations. Weighting method I, which utilizes only the rotational character of a mode in determining its contribution to absorption, performed slightly better than method II, a more exact treatment which incorporates the extent to which a mode is IR-active. Method III, which includes the contributions of induced dipoles, was successful in describing the influence of induced dipoles on the far-infrared spectra of the model liquids. The contribution to absorption of unstable modes with imaginary frequencies was found to be significant at low frequencies, and was treated by a simple approximation. Agreement between INM theory and the MD analysis was quite good for chloroform and ammonia, but less so for water. Agreement with experimental data in the range of 4–100 cm−1 was generally poor, but no worse than that for the MD calculations, primarily reflecting the simple intermolecular potentials used rather than the computational method itself. © 1997 American Institute of Physics.

Notizen: The vibrational fine structure of the prominent C1s-π* absorption band of ethylene and some of its isotopomers has been investigated theoretically with the aid of a specific (linear) vibronic coupling model. The presence of two equivalent C1s levels gives rise to two nearly degenerate electronic states of g and u symmetry, respectively, which can interact vibronically via the (planar) antisymmetric C–H stretching and bending modes (ν11 and ν12). In addition to these states of immediate interest, the present model comprises three more pairs of "effective" states at higher energy allowing one to describe the excitation of additional asymmetric (nonplanar) modes as a result of linear vibronic coupling. To a good approximation the intrapair vibronic coupling can be eliminated by using a representation in terms of localized C1s hole states. As a further result, the in-plane and out-of-plane modes become separable. The required vibronic coupling constants have been determined with the aid of ab initio calculations at various nuclear conformations using a second-order polarization propagator method. After slight readjustment of some of the parameters, the calculated spectral profiles are found to be in excellent agreement with the experimental findings. The theoretical spectra reflect strong excitation of nontotally symmetric modes (and the concomitant symmetry lowering), involving mainly the planar C–H stretching mode ν11 and the out-of-plane C–H bending mode ν8. While the planar distortion is a consequence of the equivalent core levels, the nonplanar symmetry lowering can be rationalized in chemical terms as a rehybridization effect in which the sp2 bonding scheme in the ethylene ground state is changed to sp3 in the excited state. © 1997 American Institute of Physics.

Notizen: The FFT-symplectic integrator (SI) scheme devised for solving the wave packet propagation problem is applied to stationary reactive-scattering problems. In order to relate the stationary problem to the time-dependent problem, a class of Schrödinger equation with an inhomogeneous wave source term is introduced. By using the equivalence between the stationary scattering eigenstate and the equilibrium state of the inhomogeneous Schrödinger equation, the scattering eigenstates can be computed by integrating the inhomogeneous Schrödinger equation with the FFT-SI scheme. A Gaussian wave source is proposed as an efficient wave source exhibiting rapid relaxation toward the eigenstate. Our method is tested by a one-dimensional example which has an analytical solution, and great numerical accuracy is confirmed. It is further examined by an example of time-dependent scattering and by a two-dimensional example of chaotic tunnel-scattering. © 1997 American Institute of Physics.

Notizen: The electronic structure of the GdF molecule is investigated using all-electron Dirac–Fock–Roothaan calculations. It is found that, in the ground state, the Gd atom transfers a 5d electron to the 2p spinors of the F atom, so that the molecule is ionic, having the configuration of Gd+F−. However, the molecule is not purely ionic, since the electrostatic field produced by Gd+ and F− causes the spinor energies of F 2s and one of the Gd 5p to be almost energetically degenerate so that these spinors strongly mix with each other and form covalent bonds. The electrostatic field also causes a large energy lowering for one of the 4f spinors, giving further stability to GdF. The 4f electrons of Gd should be regarded as valence electrons. The lower excited states and positively and negatively ionized states are found to be roughly described by Gd atomlike excitations, ionization, and electron attachments. © 1997 American Institute of Physics.

Notizen: We compare two time-dependent methods (time-dependent Hartree and time-dependent density functional methods) with a time-independent density functional method for the calculation of the frequency dependent polarizability and resulting absorption spectrum of two interacting quantum confined particles (quantum dots). The system is examined within the dipole approximation and the methods are evaluated in terms of the optical absorption spectrum. The spectral noise generated by time-dependent methods is a sensitive measure of the degree of broken correlation between the quantum degrees of freedom and the time-dependent density functional method may help to quantify the efficacy of correlation-exchange potentials that are used in density functional models. With respect to the quantum confinement issue, we find that increasing the interaction energy between nearest neighbor quantum dot sites represented in our model tends to shift absorption intensity to higher energy transitions. © 1997 American Institute of Physics.

Notizen: Density functional (DF) calculations have been performed for lithium clusters Lin and their monoxides LinO with up to ten atoms. There are numerous stable structures, and new isomers have been found in each family. The structural patterns of the homonuclear and oxide clusters are quite distinct. The combination of DF calculations with molecular dynamics (MD) sheds light on the observed pseudorotation of Li3 and Li5. We compare with available experimental data and discuss the bonding and structural patterns in the clusters and their oxides, which are often described as "hyperlithiated." © 1997 American Institute of Physics.

Notizen: Industrially important plasmas offer a variety of complicated molecular processes that benefit from predictive quantum chemical techniques. Ab initio coupled-cluster and MBPT methods are used to characterize structures, vibrational frequencies, ionization potentials, electron affinities, and excited states for the main fragments in the BCl3 plasma, i.e. BCl3, BCl2, BCl, and their anions and cations for which few experimental results exist. The excited, electron attached, and ionized states are calculated by employing the equation-of-motion coupled cluster (EOM-CC) method. Recent results from a photofragmentation study and an electron collision experiment are analysed based on the calculated results. Some features of the potential energy surfaces of excited states of BCl2 are discussed in order to explain the origin of the experimental fluorescence spectrum. We also consider possible microscopic processes with low energy, such as the formation and destruction of neutral and ionic species, decomposition paths, and the role of each fragment. While decomposition through transient states of BCl3− by electron attachment is the most probable path for low-energy electron attachment, decomposition through excited states of BCl3 can play a role only when there is no other way to make the BCl3+ ion.© 1997 American Institute of Physics.

Notizen: Relaxation in supercooled liquids is formulated from the instantaneous normal modes (INM) point of view. The frequency and temperature dependence of the unstable, imaginary frequency lobe of the INM density of states, 〈ρu(ω,T)〉 (for simplicity we write ω instead of iω), is investigated and characterized over a broad temperature range, 10≥T≥0.42, in the unit density Lennard-Jones liquid. INM theories of diffusion invoke Im-ω modes descriptive of barrier crossing, but not all imaginary frequency modes fall into this category. There exists a cutoff frequency ωc such that modes with ω〈ωc correspond to "shoulder potentials," whereas the potential profiles include barrier-crossing double wells for ω〉ωc. Given that only modes with ω〉ωc contribute to diffusion, the barrier crossing rate, ωh, and the self diffusion constant D, are shown to be proportional to the density of states evaluated at the cutoff frequency, 〈ρu(ωc,T)〉. The density of states exhibits crossover behavior in its temperature dependence such that the exponential T-dependence of D(T) crosses over from Zwanzig–Bassler exp(−E2/T2) behavior at low T to Arrhenius exp(−E/T) behavior at high T; the exponential may be too weak to be observed, in which case D(T) is a power law. Based on the properties of LJ, a general INM description of strong and fragile liquids is presented, with a physical interpretation in terms of the "landscape" of the potential energy surface. © 1997 American Institute of Physics.

Notizen: For infrequent-event systems, transition state theory (TST) is a powerful approach for overcoming the time scale limitations of the molecular dynamics (MD) simulation method, provided one knows the locations of the potential-energy basins (states) and the TST dividing surfaces (or the saddle points) between them. Often, however, the states to which the system will evolve are not known in advance. We present a new, TST-based method for extending the MD time scale that does not require advanced knowledge of the states of the system or the transition states that separate them. The potential is augmented by a bias potential, designed to raise the energy in regions other than at the dividing surfaces. State to state evolution on the biased potential occurs in the proper sequence, but at an accelerated rate with a nonlinear time scale. Time is no longer an independent variable, but becomes a statistically estimated property that converges to the exact result at long times. The long-time dynamical behavior is exact if there are no TST-violating correlated dynamical events, and appears to be a good approximation even when this condition is not met. We show that for strongly coupled (i.e., solid state) systems, appropriate bias potentials can be constructed from properties of the Hessian matrix. This new "hyper-MD" method is demonstrated on two model potentials and for the diffusion of a Ni atom on a Ni(100) terrace for a duration of 20 μs. © 1997 American Institute of Physics.

Notizen: The torsional potential function for butane was probed theoretically using increasingly complete basis sets (up to 840 functions) and treatments of electron correlation [up to the CCSD(T) method] until it was shown that the sequence of relative energies approached convergence. The Schrödinger limit in the Born–Oppenheimer approximation was thus estimated. The equilibrium energies relative to the anti conformation (ΔEe) as obtained by this focal-point extrapolation were 0.62, 3.31, and 5.51 kcal mol−1 for the geometrically optimized stationary points having carbon backbone torsion angles of 64.8°, 119.6°, and 0°, respectively. The final prediction of the anti–syn difference is 5.40±0.15 kcal mol−1. Some consequences of this result are discussed. © 1997 American Institute of Physics.

Notizen: The equilibrium properties of an isolated ring polymer are studied using a Born–Green–Yvon (BGY) integral equation and Monte Carlo simulation. The model polymer is composed of n identical spherical interaction sites connected by universal joints of bond length σ. In particular, we study rings composed of up to n=400 square-well spheres with hard-core diameter σ and well diameter λσ (1≤λ≤2). Intramolecular site–site distribution functions and the resulting configurational and energetic properties are computed over a wide range of temperatures for the case of λ=1.5. In the high temperature (good solvent) limit this model is identical to a tangent-hard-sphere ring. With decreasing temperature (worsening solvent) both the radius of gyration and the internal energy of the ring polymer decrease, and a collapse transition is signaled by a peak in the single ring specific heat. In comparison with the Monte Carlo calculations, the BGY theory yields quantitative to semiquantitative results for T(approximately-greater-than)Tθ and is qualitatively accurate for T(approximately-less-than)Tθ, where Tθ is the theta temperature. The thermal behavior of an isolated square-well ring is found to be quite similar to the behavior of an isolated square-well chain. The BGY theory indicates that rings and chains have comparable theta and collapse transition temperatures. In the low temperature limit (collapsed state) the microscopic structure of rings and chains becomes nearly identical. © 1997 American Institute of Physics.

Notizen: We present a theoretical formulation for the solvent force constant and the asymmetry parameter which characterize the free energy profile governing the electrical potential fluctuations of the solvent molecules around solutes. This formulation allows the calculation of these parameters via the extended reference interaction site method, an integral equation theory developed for molecular liquids and solutions. Since the method is inherently a nonlinear theory, we can shed some light on the possible nonlinear effects in the electrical potential fluctuations. The nonlinear effects in the solvent force constant and the validity of the linear approximation will be examined by a series of calculations for monatomic and diatomic solutes immersed in a simple dipolar solvent. The origin of the solute-dependent solvent force constant is discussed. © 1997 American Institute of Physics.

Notizen: The structure of a physisorbed carbonyl sulfide (OCS) monolayer on a well-defined NaCl single crystal surface has been studied by helium atom scattering (HAS), low energy electron diffraction (LEED) and polarized Fourier-transform infrared spectroscopy, in the temperature range 30–100 K. The monolayer growth proceeds via large islands and both HAS and LEED indicate a (2×1) commensurate structure with two molecules per unit cell. The binding energy is estimated to be about 0.26 eV from desorption experiments. By contrast, the occurrence of three intense and three weaker absorption peaks in the infrared spectrum of the asymmetric stretching vibration ν3 is difficult to reconcile with the conclusions drawn from HAS and LEED. Therefore semi-empirical potential calculations are performed to determine the monolayer structure. Energy minimization rules out a high molecular density for the layer, and shows the existence of two isoenergetical (2×1) and (2×2) phases with respectively two and four molecules per unit cell, which differ only by the orientations of the molecules. Because of their similar geometry, these two structures cannot be discriminated by HAS and LEED, but the mixing of these two phases with different vibrational couplings between the admolecules provides a reasonable interpretation of the main features of the infrared spectrum. © 1997 American Institute of Physics.

Notizen: The jet-cooled laser induced fluorescence excitation spectrum of the A˜ 1A′′−X˜ 1A band system of DSiF has been observed using the pulsed discharge jet technique. Vibrational analysis of the spectrum yielded upper state harmonic vibrational frequencies of ω1=1322, ω2=444, and ω3=867 cm−1. Vibronic bands involving all of the upper state fundamentals of HSiF and DSiF have now been rotationally analyzed, allowing a determination of the excited state equilibrium structure as re′(SiH)=1.526±0.014 Å, re′(SiF)=1.597±0.003 Å, and θe′(HSiF)=115.0±0.6°. The harmonic frequencies and centrifugal distortion constants were used to obtain harmonic force fields and average (rz) structures for the ground and excited states. The ground state average structure was used to estimate the equilibrium structure of re″(SiH)=1.528±0.005 Å, re″(SiF)=1.603±0.003 Å, and θe″(HSiF)=96.9±0.5°. © 1997 American Institute of Physics.

Notizen: The explicit result for the Mueller matrix for the sum of single and double scatterings is obtained in a nonpolar binary liquid mixture composed of a chiral solute and an achiral solvent are obtained with the aid of the Ornstein–Zernike form for the correlation function of concentration fluctuations. The explicit result of the matrix shows that the law of reciprocity does not hold in the mixture. The scattering phenomena for six kinds of completely polarized light in a nonpolar binary liquid mixture are different from those in a pure chiral fluid because of the failure of the law and the large effect of concentration fluctuations. The scattering phenomena are in detail discussed in the region far from and near the critical point. © 1997 American Institute of Physics.

Notizen: A simple cubic lattice model and a matrix formalism are used to calculate the segment density distribution and structural properties of monolayers consisting of end-attached chains and solvent molecules. The chain stiffness, the nearest-neighbor bond correlations, and the intermolecular interactions are taken into account. The calculations are performed for chains of various lengths, from short to polymer brushes. The main difference between the present treatment and those already available consists in the more compact formulation of the basic equations. The incorporation of the nearest-neighbor bond correlations, or higher values of the interaction parameter results in a higher average segment density and a narrower distribution profile of the chain free ends. With decreasing chain length, the effect of the bond correlations becomes less important. The model allows us to calculate the density profiles of the lateral, forward, and backward bonds as a function of the layer number, and to calculate the orientational probability for a bond to be lateral, forward, or backward as a function of the bond number counted from the attached end. © 1997 American Institute of Physics.

Notizen: A new version of the filter diagonalization method of diagonalizing large real symmetric Hamiltonian matrices is presented. Our previous version would first produce a small set of adapted basis functions by applying the Chebyshev polynomial expansion of the Green's function on a generic initial vector χ. The small Hamiltonian, H, and overlap, S, matrices would then be evaluated in this adapted basis and the corresponding generalized eigenvalue problem would be solved yielding the desired spectral information. Here in analogy to a recent work by Wall and Neuhauser [J. Chem. Phys. 102, 8011 (1995)] H and S are computed directly using only the Chebyshev coefficients cn=〈χ|Tn(H(circumflex))|χ〉, calculation of which requires a minimal storage if the H(circumflex) matrix is sparse. The expressions for H and S are analytically simple, computationally very inexpensive and stable. The method can be used to obtain all the eigenvalues of H(circumflex) using the same sequence {cn}. We present an application of the method to a realistic quantum dynamics problem of calculating all bound state energies of H3+ molecule. Since the sequence {cn} is the only input required to obtain all the eigenenergies, the present method can be reformulated for the problem of spectral analysis of a real symmetric time signal defined on an equidistant time grid. The numerical example considers a model signal C(tn)=∑kdk cos(tnωk) generated by a set of N=100 000 frequencies and amplitudes, (ωk,dk). It is demonstrated that all the ωk's and dk's can be obtained to very high precision using the minimal information, i.e., 200 000 sampling points. © 1997 American Institute of Physics.

Notizen: The theory of transport in highly inhomogeneous systems, developed recently by Pozhar and Gubbins, and the nonequilibrium molecular dynamics (NEMD) technique are employed to study the viscosity of WCA fluids confined in narrow slit pores of width 5.1 and 20σ at reduced densities ρσ3 of 0.422–0.713. Calculated quantities include the equilibrium and nonequilibrium density profiles, equilibrium pair correlation functions, flow velocity profiles, and the viscosity profiles. NEMD simulation results are compared with the theoretical predictions. The agreement is good except for the region within one molecular diameter from the walls. The viscosity was found to vary with position across the pore. © 1997 American Institute of Physics.

Notizen: Molecular nitrogen is well known for its chemical inactivity. Experimental results for grazing incidence N2 scattering from Pd(111) surfaces in the keV range also reveal negligible influences of electronical processes on molecular fragmentation. Therefore, we carry out an appropriate classical treatment of this system. The N2–Pd(111) interaction is mediated by an analytical six-dimensional potential energy surface, based on ab initio density-functional-theory calculations and empirical data. The molecule-surface interaction seems to be strongly influenced by the azimuthal direction of incidence as well as the molecular axis orientation. Particularly, the fragmentation is found to be mainly due to vibrational excitation for highly indexed azimuthal directions, whereas for incidence along lowly indexed directions rotational excitation is more important. © 1997 American Institute of Physics.

Notizen: The change of the MgO(001) surface during the adsorption and thermal desorption of formic acid was investigated by temperature programmed desorption (TPD) and infrared-visible sum-frequency generation SFG spectroscopy. The TPD results revealed that the feature of desorption signals changed as the adsorption/thermal desorption process was repeated, despite no significant change of the low energy electron diffraction (LEED) pattern. The surface formate, produced by dissociative adsorption of formic acid, decomposed at 560 K and 700 K, giving H2O+CO and H2+CO2 as primary and minor products, respectively, but the broad desorption peak of H2O started to appear at 300 K. The 560 K peak appeared from an early stage, while the 700 K peak gained intensity as the experiment was repeated and the density of defects increased. The adsorption site for the 700 K peak was ascribed to the catalytic reaction of magnesia powder. The SFG spectra gave three resonance peaks of CH stretching bands at 2850 cm−1, 2870 cm−1, and 2920 cm−1 with a different dependence on the dose time, history of surface, and polarization of incident visible beam. The former two bands were assigned to the formates with bridged configuration, and the third band to the formate with unidentate configuration. The polarization dependence suggested that the CH bonds of bridged formates stood perpendicular to the surface, and that of the unidentate formate tilted approximately 30° from the surface normal. A vibration nonresonant background signal was present on the SFG signal, showing characteristic dependence on the polarization of incident light beams, as well as the coverage and history of the surface, and was interpreted as originating from the defect sites on the surface. © 1997 American Institute of Physics.

Notizen: For the first time a numerical procedure for computing the Solomon echo amplitudes of half-integer quadrupole spins (I=3/2, 5/2, 7/2, and 9/2) has been derived from a detailed analysis of the evolution of the spin system using the density operator formalism. As the first-order quadrupole interaction is taken into account throughout the experiment, consisting in exciting the spin system with two pulses either in phase or in quadrature phase and separated by a delay τ2, the results are valid for any ratio of the quadrupole coupling, ωQ, to the amplitude of the pulses, ωrf. These results are applicable to light nuclei at high magnetic field, for which the second-order quadrupole interaction and the chemical shift anisotropy are negligible. We predict (4I2−1)(2I−1)/16 echoes and their positions in the detection period τ4, (I−1/2)2 of which are allowed echoes, the others being forbidden echoes. All of these echoes are satellite-transition signals, which are superimposed on the free induction decay (FID) of the central transition following the second pulse. This means that τ2 must be short when compared to the duration of the central-transition FID. In other words, during τ2 the magnetic dipole interactions have no time to affect the evolution of the spin system. The behavior of the τ4=τ2 echoes versus the second-pulse flip angle ωrft3 is analyzed: in the hard-pulse excitation condition (ωQ/ωrf(very-much-less-than)1), the (±1/2↔±3/2) echoes generated by two pulses in quadrature phase are twice as large as those generated by two in-phase pulses; the echoes of the other transitions have comparable amplitudes. The echo amplitudes in the soft-pulse excitation condition (ωQ/ωrf=1) are also presented. © 1997 American Institute of Physics.

Notizen: Two Monte Carlo methods are used to simulate a dynamics of molecules of a foreign gas A, highly diluted in a carrier gas C. Diffusion in the presence of bimolecular reaction A+C→products is studied. Nonequilibrium corrections to the reaction rate constant and diffusion coefficient are calculated for a wide range of molecular mass ratio mA/mC and activation energies of the reaction. Some differences between simulation results of the two methods are observed. The deviation of the velocity distribution from the Maxwellian form is quantified by means of the fourth order cumulants. The nonequilibrium effects are most significant in the Lorentz range, that is for mA(very-much-less-than)mC. The simulation results prove that the theoretical predictions based on a perturbative solution of the Boltzmann equation are valid for not too small values of mA/mC, but are not correct in the Lorentz range. © 1997 American Institute of Physics.

Notizen: The reactions of CO+, CO2+, SO2+, NO2+, CS2+, CN+, C2N2+, and C2H3+ with H atoms and H2 molecules have been studied in a selected ion flow tube operated at (300±5) K. The H atom reactions proceed variously by the processes of atom exchange and charge transfer (when allowed), none proceed at the Langevin rate, and the rates of several of them appear to be influenced by the spin states of the product species. Most of the H2 reactions proceed by H atom abstraction and at a large fraction of the Langevin rate. © 1997 American Institute of Physics.

Notizen: A collisional reaction of a sodium cluster ion, Nan+(n=3–9), with a nitrous oxide molecule, N2O, was investigated by using a tandem mass-spectrometer equipped with four octopole ion guides. Two types of ions, NakO+ and Nap+ (p〈n), were produced by the reaction; NakO+ and Nap+ result from oxidation and direct collisional dissociation of the parent cluster ion, Nan+, respectively. The absolute total cross section and partial cross sections for the NakO+ (oxidation) and Nap+ (direct dissociation) production were measured as a function of the collision energy of a given parent cluster ion, Nan+, in the energy range up to 5 eV (center-of-mass frame). The cross sections and their collision-energy dependence show that the oxidation proceeds when electron harpooning from Nan+ to N2O occurs and otherwise the direct collisional dissociation proceeds. © 1997 American Institute of Physics.

Notizen: Relativistic complete-active space self-consistent field and multireference configuration interaction computations including spin–orbit coupling are made on the TaC2+ cation, the most abundant of metal carbides observed in direct laser vaporization and Knudsen effusion–mass spectrometric experiments. The suggested mechanism of TaC2+ formation via recombination reactions between Ta+ and C2 is consistent with the calculated potential energy curves for the low-lying electronic states. Two nearly degenerate unsymmetrical linear (Ta–C–C+) and symmetrical C2v(C–Ta–C+) isomers suggest that the products can include a mixture of both isomers. © 1997 American Institute of Physics.

Notizen: The chlorine tetroxide radical and its anion have been investigated with unrestricted Hartree–Fock, density functional and hybrid Hartree–Fock/density functional methods, perturbation theory, and coupled cluster methods. It is predicted that the minimum on the ClO4 radical potential energy surface corresponds to the 2B2 ground electronic state, a C2v-symmetry structure. The C3v structure identified experimentally lies a few kcal mol−1 higher energetically. A Cs symmetry structure lies very close energetically to the C2v minimum at some levels of theory. Adiabatic electron affinities, equilibrium geometries, harmonic vibrational frequencies, infrared intensities, isotopic shifts, and dipole moments are presented for all methods. Comparisons with recent experimental vibrational data for the radical are made. It is predicted that the C2v equilibrium geometry of the ground state is Re(Cl–O1)=1.502 Å, Re(Cl–O2)=1.424 Å, θe1=93.6° and θe2=114.2°. The adiabatic electron affinity of the ClO4 radical is predicted to lie near 5.2 eV. © 1997 American Institute of Physics.

Notizen: The theory of relaxed density matrices has been developed for the calculation of second-order response properties at third-order Møller–Plesset (MP3) and linearized coupled cluster double excitation (L-CCD) levels of theory. The ensuing algorithm is applied to the determination of the isotropic and anisotropic nuclear magnetic shielding constants and magnetizabilities for thirteen molecules (H2, N2, F2, HF, CO, HCN, HNC, H2O, NH3, H2O2, HCHO, CH4, and HCCH). The method uses conventional gauge-variant orbitals but, by using large basis sets, produces results which are equivalent to those found with gauge-including orbitals. In general the L-CCD values of the magnetizabilities are consistent with those obtained with multiconfigurational self-consistent-field (MCSCF) methods for molecules which have been treated by this method. For the nuclear shieldings, when there is a difference between L-CCD and MP3, the former gives results closer to the coupled-cluster singles and doubles level treatment augmented with a perturbation correction for connected triple excitations [CCSD(T)] which is our reference point. Again the results for the shieldings at the L-CCD level are quite good. We also use the paramagnetic components of the shieldings and magnetizabilities to determine the spin-rotation constants and rotational g tensors, respectively. These quantities are important since they may be compared more directly with experiment than the magnetizabilities and shieldings. © 1997 American Institute of Physics.

Notizen: For the initially formed C3H2 collision complexes of molecular beam experiments ab initio calculations are presented. Resolving energetics and properties of these intermediates is essential for the understanding of the reaction of C(3P) with C2H2 to form interstellar cyclic and linear isomers of C3H. Computed reaction energies agree with results from molecular beam experiments. The combination of crossed molecular beam experiments and ab initio calculations allows us to identify two reaction channels for the carbon–hydrogen exchange and to explain astronomical observations of a higher c-C3H to 1-C3H ratio in dark clouds as compared to hotter envelopes of carbon stars. © 1997 American Institute of Physics.

Notizen: Extensive Gibbs ensemble Monte Carlo (GEMC) simulations of a rigid molecule model of C60, characterized by a deeply attractive short-ranged interaction potential, are performed with the aim to establish the effect of the system size on the existence and location of the liquid–vapor binodal line and of its critical point. The results obtained with N=300, 600, and 1500 particles indicate that the position and the overall shape of the binodal is only minorly influenced by finite size effects. The estimated critical temperature and density at the different N fall in the ranges 1920–1940 K, and 0.4–0.45 nm−3, respectively. The results are discussed by making reference to previous studies of finite size effects in GEMC simulations. The GEMC predictions are also compared with previous computer simulation and theoretical calculations for the same model fluid. The agreement is on the whole satisfactory for both the liquid–vapor coexistence line and the critical point parameters. On the basis of previously determined freezing lines of C60, and of the actual binodal line, different estimates of the location of the triple point are also made. Triple point temperatures are found, in any case, definitely lower (by at least 150 K) than the critical temperatures, thus confirming the existence of a relatively narrow liquid phase region in the phase diagram, as predicted in previous molecular dynamics and theoretical works. The existence of such a liquid phase for the adopted model potential is discussed and assessed in the more general framework of liquid–vapor coexistence conditions in fluids interacting through short-ranged forces. The possibility to get the liquid phase of "real life" C60, hitherto not observed experimentally, is also discussed in connection with recent high temperature experimental results on fullerite samples. © 1997 American Institute of Physics.

Notizen: Steady state photoconduction was studied in optically thin spin coated and optically thick solution grown films of an oligo-para-phenylenevinylene sandwiched between indium tin oxide (ITO) and aluminum (Al) contacts. Oligomers represent model systems for polyphenylenevinylene that do not contain structural irregularities and impurities always present in polymers. Upon irradiation through the positively biased ITO contact, dissociation of excited states via electron transfer to the ITO prevails. It leads to a field and temperature dependent photocurrent that is symbatic with the absorption spectrum. The efficiency of this process increases with excess energy of the primarily excited state. Photocurrents due to bulk photoionization are observed upon irradiating through the positively biased Al contact. They result from temperature and field assisted dissociation of geminate e–h pairs generated either by field induced break-up of molecular singlet states or by direct charge transfer transition. © 1997 American Institute of Physics.

Notizen: The temporal dependence of the time correlation function detected by quasielastic light (QEL) scattering in solution consisting of two homopolymers with arbitrary hydrodynamic and thermodynamic properties is investigated. New results appropriate for the ternary polymer solution in various concentrations are obtained. These new results should facilitate the analysis of the QEL scattering data of a ternary solution in any concentrations. Information about tracer diffusion coefficients of the constitutive polymers and their excluded volume interaction parameter is shown to be obtainable from the QEL experiment of the ternary polymer solution. Applications to ternary polymer solutions with one polymer isorefractive with the solvent, solutions with two polymers differing only in molecular weight, and solutions with two polymers having arbitrary refractive index increments are discussed. Clarification in regard to the ternary solution under the zero average contrast (ZAC) condition is also made. It is shown that under the ZAC condition, the autocorrelation function of the composition fluctuation in general decays with two exponentials, even at 1:1 symmetric composition. © 1997 American Institute of Physics.

Notizen: A differential in the relaxation rates of the ice surface and subsurface during Ostwald ripening near 138 K is the basis for a new approach to determination of the separate surface and subsurface vibrational spectra of ice nanocrystals. The resulting FT-ir spectrum of the ice subsurface matches that previously obtained from adsorbate-induced relaxation of the subsurface, confirming both the form of the surface and subsurface spectra, and the disordered nature of the surface of the nanocrystals. From these data, together with the results of simulation studies, ice emerges as the only molecular crystalline substance for which the surface and subsurface spectra are known and assigned. Semiquantitative rates of the structural relaxations near the surface, for bare ice and ice coated with the adsorbate SO2, have also been determined. Finally, spectral data are presented that track the insertion of SO2 into the vulnerable strained H bonds common to simulated structures of the ice surface. © 1997 American Institute of Physics.

Notizen: Accurate all-electron Dirac–Fock–Roothaan calculations were performed for the 1Σ+ ground state of ThO using Gaussian basis sets of Th (24s20p16d12f)/[19s15p12d5f] and O (10s6p1d)/[7s4p1d]. The relativistic effects in ThO were found to slightly extend the bond length re, to increase the dissociation energy De, and to decrease the harmonic frequency ωe. © 1997 American Institute of Physics.

Notizen: Systematic analysis is carried out for molecular phase in (ω1,ω3) coherent control scheme. Two mechanisms for nonzero molecular phase are proposed. It is shown that the imaginary part of the three-photon transition operator, which is due to the contribution of the intermediate continuua, can give different phase factors for the one- and three-photon transition amplitudes, yielding very important effects of molecular phase in (ω1,ω3) coherent control. We also show that the interactions between the final continuum states can result in nonzero molecular phase, when the intermediate states are discrete. These findings are discussed in relation to Gordon and co-workers' recent observations [Zhu et al., Science 270, 77 (1995)] on phase lag. © 1997 American Institute of Physics.

Notizen: Line intensities in the (0,0) band of NF(b 1Σ+→X 3Σ−) spin forbidden emission have been observed in a microwave apparatus. Deduced line strengths of the OP and SR well resolved branches are shown differently, in contrast with the theoretical prevision, and are interpreted by taking into account some peculiarities of the NF radical. An intermediate (a) and (b) Hund case is seen to apply toOP and SR in the whole rotational range examined. Accordingly, a reliable value for the effective transition momentμe is evaluated from the experiment. © 1997 American Institute of Physics.

Notizen: Reactions of laser-ablated Cr atoms with O2 gave a very strong, sharp 965.4 cm−1 band and weak, sharp 1869.7, 984.3, 914.4, 846.3, 716.2, and 643.1 cm−1 bands. The 1869.7, 965.4, and 914.4 cm−1 bands track together on annealing, show 52Cr, 53Cr, 54Cr isotopic splittings appropriate for a single Cr atom and triplets with statistical 16,18O2 for two equivalent O atoms, and are assigned to the ν1+ν3, ν3 and ν1 modes of the bent (128°±4°) chromium dioxide OCrO molecule. The 984.3 cm−1 band shows chromium isotopic splittings for two Cr atoms and 16,18O2 components for two O atoms, and is attributed to the bent CrOCrO molecule. The weak 846.3 cm−1 band exhibits proper oxygen isotopic behavior for CrO and is redshifted 39 cm−1 from the gas-phase value, the maximum shift observed for a first row transition metal monoxide. The sharp 716.2 and 643.1 cm−1 bands track together; the former reveals Cr isotopic splittings for two Cr atoms and the latter 16,18O2 splittings for two sets of dioxygen subunits; the branched-puckered-ring dimer O(Cr2O2)O is identified. Annealing produces new bands due to CrOO, CrO3, Cr(OO)2 and the ring dimers (Cr2O2) and (Cr2O2)O, which are identified from isotopic shifts and splitting patterns. © 1997 American Institute of Physics.

Notizen: We investigate the capture of lithium atoms by a beam of large cold helium clusters and subsequent ion production by electron impact. Li atoms are efficiently picked up by the He droplets, although with a cross section significantly lower than that for other atoms and molecules. The mass spectrum reveals the presence of Li atom and dimer ions, as well as the weakly bound complex HeLi+, confirming that capture by He-cluster beams can be used for efficient soft ionization of fragile species. The electron-energy dependence of the Li+, Li2+, and HeLi+ yield shows that they are formed primarily by Penning ionization in a collision with a metastable He atom in the droplet. This leads to the conclusion that lithium metal atoms are not submerged in the helium clusters but locate on the surface, corroborating theoretical predictions for bulk helium surfaces and spectroscopic measurements. © 1997 American Institute of Physics.

Notizen: The determination of rate constants for chemical reaction systems is of considerable practical importance. This paper introduces a temporal inversion method for parameter identification of dynamical systems, and butene isomerization is used to illustrate the method. The rate constants are artificially treated as temporal functions and an explicit equation describing the time evolution of the rate constants is obtained by minimizing the error along the reaction course. With appropriate experimental data the unknown rate constants are guided from arbitrary initial values to their best values for the available experiments at the final time. The method leads to an iterative algorithm. Numerical results show that accurate estimates of all the rate constants can be obtained. © 1997 American Institute of Physics.

Notizen: Within the universal integral formalism of encounter theory, we described (a) the bimolecular charge recombination that follows the geminate one beginning with photoionization and (b) bimolecular quenching of singlet and triplet excitations parallel to an inter-system crossing (ISC). In both cases the new features appear which represent either the geminate ion pair density (positive) or pair vacancies density (negative), which accounts for singlets quenched before ISC. It was justified that the shortage of near-contact excitations resulting from static singlet quenching may be built into the initial conditions for subsequent evolution of triplets and lead to the "anti-Smoluchowski time behavior" of their annihilation rate constant. However, only an integral theory can account for the same effect when singlet quenching is diffusion accelerated. © 1997 American Institute of Physics.

Notizen: We propose a common rigorous foundation to the classical collision theory and that of the classical activated complex. Based on the notion of activated complex, this foundation relies on a stochastic approach showing up the different influence factors of a chemical reaction. The thermodynamic formulation is obtained here by assuming the exponential statistical distribution within each stable chemical state. The general model we obtain yields two stochastic formulations called the stochastic transition state theory (denoted STST) and the stochastic activated collision theory (denoted SACT) respectively, depending on whether the rate of the reaction is of the same scale as, either the rate of passing over the potential energy barrier, for the STST, or the rate of reaching the activated complex state (which is a generalization of the collision rate), for the SACT. The modeling is first done in the case of a closed small system undergoing stochastic changes of chemical state and then, by extension, in the frame of classical chemical kinetics. Some properties of these two theories are studied. Furthermore the formulation is applied to the stochastic example of RNA structure changes and to the classical simple example of a reaction between two atoms A and B to form the diatomic molecule AB. © 1997 American Institute of Physics.

Notizen: Raman excitation profiles for twelve vibrational modes of the solvatochromic dye betaine-30 have been measured in CH3OH and CD3OD solutions at wavelengths that span the S0→S1 charge transfer transition. Though the absorption spectra of the dye are the same in the protonated and deuterated forms of the solvent, Raman cross-sections for all modes were found to be generally lower in the CH3OH solution than inCD3OD. The time-dependent theory of Heller was applied to model the absorption and Raman profiles, and both mono- and bi-exponential solvent relaxation were considered in order to account for solvent induced electronic dephasing. The two models lead to different physical pictures for the relevant solvent dynamics, but in either case the amplitude of solvent dephasing is reduced in deuterated compared to protonated methanol. The effect is interpreted in terms of stronger solvent–solute and solvent–solvent hydrogen bonding in deuterated methanol solution. Comparing to previous results for betaine-30 Raman cross-sections in acetonitrile [Y. Zong and J. L. McHale, J. Chem. Phys. 106, 4963 (1997)], it is concluded that slower solvent dynamics and perturbations to the electronic structure of betaine-30 lead to larger Raman intensities in methanol. The data suggest that solvent dynamics depend strongly on the electronic state of betaine-30. © 1997 American Institute of Physics.

Notizen: We present the investigations of nonadiabatic scattering processes (reactive as well charge-transfer) occurring in B++H2 reaction in the gas phase on the triplet electronic surfaces utilizing a mixed quantum-classical approach to scattering of three particle systems in hyperspherical coordinates. The time-dependent Schrödinger equation is solved in diabatic representation using wave packet propagation method on a grid in two quantum dimensions. The potential-energy surfaces have been obtained using the valence-bond diatomics-in-molecule approach. © 1997 American Institute of Physics.

Notizen: We present a new integral equation formulation of the polarizable continuum model (PCM) which allows one to treat in a single approach dielectrics of different nature: standard isotropic liquids, intrinsically anisotropic medialike liquid crystals and solid matrices, or ionic solutions. The present work shows that integral equation methods may be used with success also for the latter cases, which are usually studied with three-dimensional methods, by far less competitive in terms of computational effort. We present the theoretical bases which underlie the method and some numerical tests which show both a complete equivalence with standard PCM versions for isotropic solvents, and a good efficiency for calculations with anisotropic dielectrics. © 1997 American Institute of Physics.

Notizen: A new parametric crossover model for the phase behavior of a binary mixture is presented that corresponds to the Leung–Griffiths model in the critical region and is transformed into the regular classical expansion far away from the critical point. The model is optimized to, and leads to excellent agreement with, isothermal vapor–liquid equilibrium data for dilute aqueous solutions of sodium chloride by Bischoff and co-workers. It then accurately predicts constant-composition phase equilibrium loci as measured by independent workers. This crossover model is therefore capable of representing the thermodynamic surface of ionic solutions in a large range of temperatures and densities around the critical points of vapor–liquid equilibrium.

Notizen: We have calculated the viscosities of a biaxial nematic liquid crystal phase of a variant of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by performing molecular dynamics simulations. The equations of motion have been augmented by a director constraint torque that fixes the orientation of the directors. This makes it possible to fix them at different angles relative to the stream lines in shear flow simulations. In equilibrium simulations the constraints generate a new ensemble. One finds that the Green–Kubo relations for the viscosities become linear combinations of time correlation function integrals in this ensemble whereas they are complicated rational functions in the conventional canonical ensemble. We have evaluated these Green–Kubo relations for all the shear viscosities and all the twist viscosities. We have also calculated the alignment angles, which are functions of the viscosity coefficients. We find that there are three real alignment angles but a linear stability analysis shows that only one of them corresponds to a stable director orientation. The Green–Kubo results have been cross checked by nonequilibrium shear flow simulations. The results from the different methods agree very well. Finally, we have evaluated the Miesowicz viscosities [D. Baalss, Z. Naturforsch. Teil A 45, 7 (1990)]. They vary by more than 2 orders of magnitude. The viscosity is consequently highly orientation dependent. © 1997 American Institute of Physics.

Notizen: We have analyzed the near-infrared band of NO3 observed at 7602 cm−1 by using diode laser spectroscopy. Most of the spectral lines were recorded using source-frequency modulation. Zeeman modulation was found useful in selectively detecting some Q branch lines, which provided us with a clue to the assignment of the observed spectra. The band satisfied selection rules for a parallel band and was thus ascribed to a 2A1″–2A2′ vibronic component associated with the 2E′′–X˜ 2A2′ electronic transition, namely, to a transition from the ground vibronic state to the A1″ vibronic state resulting from excitation of the degenerate in-plane bending mode in the 2E′′ electronically excited state manifold. The band was almost free of perturbations, except for some K=6 lines. The least-squares analysis of 581 assigned lines led to molecular parameters of the upper state, where ground-state parameters were fixed to those obtained from the infrared study previously reported [K. Kawaguchi, E. Hirota, T. Ishiwata, and I. Tanaka, J. Chem. Phys. 93, 951 (1990)]. The upper-state B rotational constant gave the effective N–O distance of 1.271 Å, which is to be compared with 1.240 Å in the ground vibronic state. The εbb spin–rotation interaction constant of the upper state was close in magnitude to that in the ground vibronic state, but of opposite sign. This observation indicates that the spin–rotation interaction is primarily caused by that between the 2E′′ excited and the ground electronic states. © 1997 American Institute of Physics.

Notizen: We discuss the observation of coherent nuclear oscillations in chemical and biological systems subsequent to excitation with an ultrashort laser pulse. The experimentally observed coherent signals arising from rapid, photon initiated, chemical reactions are considered separately from vibrational coherences directly excited by the (electric) field interventions of the light pulse. In order to treat the chemical reaction, a modified Landau-Zener theory is developed that is quasi-classical along the traditional reaction coordinate but fully quantum mechanical in all other degrees of freedom. In this work, we explore explicitly the effects of electronic state coupling, electron-nuclear coupling, reaction velocity, vibrational damping and temperature on the product electronic state population development and on the coherent oscillations of the nuclear coordinates. Recent experimental studies of the ligand photolysis reaction in heme proteins are presented as examples. © 1997 American Institute of Physics.

Notizen: Neutral dissociation of superexcited states of OCS has been studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 14.2–16.8 eV. A two-dimensional spectrum exhibits noticeable features which are assigned as resulting from autoionizing transitions of excited atomic sulfur, S*, from Rydberg states converging to S+(2Do) to S+(4So). The precursor molecular states leading to S*+CO are considered to be multiple-electron-excited Rydberg states, OCS*(Dis), converging to OCS+ with 2Σ− and/or 2Δ symmetry. The electron signal counts due to autoionization of S* show enhancement at excitation photon energies for the Rydberg states, OCS*(RB), converging to OCS+(B˜ 2Σ+). These results support a predissociation mechanism for the formation of S*: conversion from OCS*(RB) to OCS*(Dis). The quantum yield for the predissociation is evaluated to be ∼1% at the photon energy corresponding to the 5sσ state of OCS*(RB). © 1997 American Institute of Physics.

Notizen: New relations among reactivity descriptors are provided within the recently introduced modified isomorphic ensemble of density functional theory. In addition, expressions for the softness and hardness kernel are derived in the canonical, grand canonical, isomorphic, and grand isomorphic ensemble. There results a new definition for the local hardness, η(r)=[∂u(r)/∂N]σ=ησ−g(r), where g(r)=[∂υ(r)/∂N]σ and σ=ρ/N is the shape factor. This identifies the local hardness as a function measuring the response of the system's external potential to a perturbation in electron number at a constant shape factor. Furthermore, it is shown that one cannot represent both local softness and local hardness unambiguously with one representation. © 1997 American Institute of Physics.

Notizen: An efficient numerical integration procedure based on the Gauss–Hermite quadrature is developed for evaluation of rovibrational Hamiltonian matrix elements in a basis of complex-scaled harmonic oscillator functions. By scaling the basis functions rather than the Hamiltonian itself, it is possible to employ molecular potential energy and coupling data directly in numerical form without first having to fit them to analytical functions. The new method is employed for the treatment of the CO B-D 1Σ+ system by employing model diabatic potentials and coupling elements from the literature. Calculations are carried out in both the original diabatic and the corresponding adiabatic representation of the electronic states. Because of the sharp oscillations in the nonadiabatic coupling functions it is found that the convergence properties in the diabatic basis are somewhat better than in the corresponding adiabatic treatment, but very good agreement is obtained between the two sets of energy and linewidth results for the lowest 11 vibrational states. Comparison is also made with earlier results for the same system obtained by employing the optical potential and close coupling methods, respectively. The second-derivative G12 coupling matrix elements are found to have an important effect on the computations in the adiabatic representation and are essential for obtaining a high level of agreement with the corresponding diabatic results. The present method is well-suited for applications based on ab initio potential energy surfaces and couplings since it requires neither that the pointwise computed data be fitted to polynomials nor that they be subjected to a diabatic transformation. © 1997 American Institute of Physics.

Notizen: Monte Carlo (MC) simulations have been carried out for supercritical mixtures of Lennard-Jones (LJ) fluids to investigate the unique behavior of excess thermodynamic functions in supercritical region obtained experimentally. It is reported that constant pressure excess molar enthalpies HpE and excess molar volumes VpE take large absolute values and show complex concentration dependencies in such regions. In this paper, HpE and VpE have been calculated for a total of 12 kinds of mixtures, each having different combinations of energy and size parameters and the combining rule for unlike interactions. It is found that such a unique thermodynamic behavior of real systems in their supercritical region can be reproduced, at least qualitatively, by simple model systems. It is also found that the unique thermodynamic behavior is closely related to the equation-of-state of pure components in their supercritical region. In order to estimate the contribution to the constant pressure excess thermodynamic functions from the equation-of-states of pure components, we have developed a data conversion method by which we can calculate a new function, excess internal energy at constant volume from HpE, VpE, and equation-of-states of pure components. The converted excess internal energies at constant volume for the present model systems become quite simple; that is, their magnitudes are reduced to be small and the concentration dependencies become symmetrical. Further analysis indicates that the large differences of molar volume and internal energy between pure components and large VpE are responsible for the unique behavior of excess functions at constant pressure. The data conversion method is also applied to the experimental data for the CO2+C2H6 system in the supercritical region. The obtained excess internal energies at constant volume also show similar simple behavior as those for the model systems. Furthermore, it is found that the complex dependence of experimental HpE on pressure is also closely related to equation-of-states of pure components. © 1997 American Institute of Physics.

Notizen: In this paper we revisit from a contemporary perspective a classic problem of polaron theory in one dimension using a new variational approach generalizing that of Toyozawa, based on delocalized trial states including mixed gobal and local exciton-phonon correlations. Polaron structure is represented by variational surfaces giving the optimal values of the complete set of exciton and phonon amplitudes for every value of the joint exciton-phonon crystal momentum κ. Characteristic small polaron, large polaron, and nearly free phonon structures are identified, and the manner in which these compete and/or coexist is examined in detail. Through such examination, the parameter space of the problem is mapped, with particular attention given to problematic areas such as the highly quantum mechanical weak-coupling regime, the highly nonlinear intermediate-coupling regime, and to the self-trapping transition that may be said to mark the onset of the strong-coupling regime. Complete energy bands are presented in illustrative cases, and the principal trends in the ground-state energy, polaron bandwidth, and effective mass are identified. The internal structure of our variational Bloch states is examined for qualities that might reflect the typical characteristics of solitons, finding some intriguing qualitative comparisons, but little that bears close scrutiny. © 1997 American Institute of Physics.

Notizen: In tetravalent network materials like quartz or ice the main interactions are the strong bonds that connect each vertex to four others and the repulsive forces that keep nonbonded vertices further apart. Computer simulations, in which those interactions are modeled by square wells, show that they are sufficient to account for the following behavior: The stability of icelike crystals, with tetrahedral bond angles, at low temperature; endothermic melting of the crystal to a denser waterlike amorph at intermediate temperatures; exothermic collapse of the supercooled crystals at high pressure, to a glass, without breaking any bonds; expansion on cooling of both the crystal and the amorph; and a line of spinodal instabilities in the waterlike supercooled amorph where the compressibility, heat capacity, and negative expansivity diverge. The spinodal is associated with correlated low-density, low-energy clusters, characterized by an excess of near tetrahedral bond angles. © 1997 American Institute of Physics.

Notizen: In this work we present a Monte-Carlo simulation of propionic acid in aqueous solution and look into the structural and thermodynamic behavior of an amphiphile at temperatures of 285, 298 and 330 K. All Monte-Carlo simulations were done with one solute and 338 water molecules, a NVT ensemble, a polarizable ab initio potential, Metropolis sampling, and periodic boundary conditions. On the technical side we show that very large statistical samples (8×108) are needed to attain reliable results and that accounting for microscopic detail is needed to find patterns that are hidden by averaging. Related to the molecular basis of hydrophobicity we found that solute–water interactions, even around the hydrophobic tail, lead to a favorable enthalpic effect, in agreement with other simulations. We also found that the tail enhances water–water interactions as compared to pure water, in agreement with the experiment. An important finding is that the anomalous solubility seems to come from an induced structure of waters neighboring the hydrophobic tail. On the contrary, at low temperatures the polar head loses its hydration shell as the water network becomes important. From a detailed analysis we propose an interplay of several factors as the reason for hydrophobicity. © 1997 American Institute of Physics.

Notizen: Rigid rodlike polymers have considerable technological importance due to their excellent mechanical properties. The polymerization kinetics of such condensation polymers are qualitatively different from flexible polymers, and exhibit significant slowing at the later stages of the reaction. This is due to the slow rotational diffusion of the molecules to an appropriate configuration for reaction. In this work we have carried out Brownian dynamics (BD) simulations to obtain the effective rate constant for reaction between rodlike molecules in the presence of diffusional limitations. The theory of Northrup et al. [J. Chem. Phys. 80, 1517 (1984)] for pairwise BD simulation of reactions is extended to the case of rodlike molecules assuming isotropic translational diffusion. The computed results are compared to exact analytical predictions. Good agreement between computation and theory is obtained over a wide range of parameter values. © 1997 American Institute of Physics.

Notizen: Exploration of the theory for spin–cavity coupling in inhomogeneously broadened spin ensembles shows that all three components of magnetization can be refocused. The conventional spin echo generated by two components of magnetization is a special case of this three component refocusing in the limit of negligible radiation damping. We demonstrate this effect by an analytical theorem, numerical simulation, and experiment. © 1997 American Institute of Physics.

Notizen: The ionization potential distribution of (CH3)2S doped in fluid krypton and xenon has been determined by means of field ionization in wide density ranges. These distributions are simulated by convoluting the field spectrum of pure (CH3)2S with the calculated polarization energy distribution of the dopant ion in the media. The density dependence of the conduction-band energy of excess electrons (V0) in the fluids is obtained from the energy shift between the experimental and simulated spectra. The resulting V0 values are compared with previous experimental results and several theoretical calculations. © 1997 American Institute of Physics.

Notizen: Two new potential surfaces for CO2–Ar, obtained principally from the spectra of van der Waals complexes, are tested against measurements of pressure broadening and of transport and relaxation properties, none of which was used in the determination of the surfaces. Pressure broadening of both infrared and Raman lines is considered. The coupled states (CS) approximation is used for all line shape calculations. Thermally averaged infrared and Raman cross sections at 523, 296, 160 and 77 K (infrared) and 295 K (Raman) show good agreement with the experimental data available. Generalized transport and relaxation cross sections are obtained via full classical trajectory and classical CS calculations. Properties tested include diffusion, viscosity and nuclear spin relaxation. They provide a different test of the surfaces and agree well with experiment. © 1997 American Institute of Physics.

Notizen: The analytical infinite order sudden (IOS) quantum theory of triatomic photodissociation, developed in paper I, is applied to study the indirect photodissociation of NOCl through a real or virtual intermediate state. The theory uses the IOS approximation for the dynamics in the final dissociative channels and an Airy function approximation for the continuum functions. The transition is taken as polarized in the plane of the molecule; symmetric top wave functions are used for both the initial and intermediate bound states; and simple semiempirical model potentials are employed for each state. The theory provides analytical expressions for the photofragment yield spectrum for producing particular final fragment ro-vibrational states as a function of the photon excitation energy. Computations are made of the photofragment excitation spectrum of NOCl in the region of theT1(13A″) ←S0(11A′) transition for producing the NO fragment in the vibrational statesnNO=0, 1, and 2. The computed spectra for the unexcited nNO==0 and excitednNO=2 states are in reasonable agreement with experiment. However, some discrepancies are observed for the singly excitednNO=1 vibrational state, indicating deficiencies in the semiempirical potential energy surface. Computations for two different orientations of the in-plane transition dipole moment produce very similar excitation spectra. Calculations of fragment rotational distributions are performed for high values of the total angular momentum J, a feature that would be very difficult to perform with close-coupled methods. Computations are also made of the thermally averaged rotational energy distributions to simulate the conditions in actual supersonic jet experiments. © 1997 American Institute of Physics.

Notizen: We have derived a theory of dynamics of dilute and semidilute polyelectrolyte solutions by explicitly considering triple screening associated with electrostatic, excluded volume, and hydrodynamic interactions. The three screening lengths corresponding to these interactions are coupled among themselves differently at different polyelectrolyte (c) and salt (cs) concentrations. We have derived expressions for the self-translational diffusion coefficient D, electrophoretic mobility μ, coupled diffusion coefficient Df, and the viscosity η of the solution by accounting for the coupling between electrostatics and hydrodynamics. In infinitely dilute solutions, we show that Zimm dynamics is applicable and D∼1/Rg, μ∼M0, and η−η0∼cRg3/M for all values of cs, where Rg and M, respectively, are the radius of gyration and molecular weight of the polyelectrolyte and η0 is the solvent viscosity. Df is derived to be M0 at low cs and to approach D at higher cs. As the polyelectrolyte concentration is increased to semidilute conditions, excluded volume and hydrodynamic interactions get progressively screened. In the Rouse regime, where hydrodynamic interaction is screened and entanglement effects are weak, we have derived expressions for the various transport coefficients. In this regime, at low cs, D∼c0M−1, Df∼c0M0, μ∼c0M0, and η−η0∼cM; at high cs, D∼c−1/2M−1, Df∼c/(c+2cs), μ∼c−1/2M0, and η−η0∼c5/4M. The crossover formulas between these asymptotic laws with numerical prefactors are derived. We have demonstrated that the Rouse law applicable to semidilute unentangled polyelectrolyte solutions at low cs is the empirical Fuoss law. The slow diffusion coefficient observed in light scattering studies of polyelectrolyte solutions is attributed to the emergence of an effective attractive interaction between similarly charged segments of topologically correlated objects such as polyelectrolytes at sufficiently high c and low cs. The consequences of entanglements at very high polyelectrolyte concentrations are briefly mentioned. The theoretical formulas derived here are in qualitative agreement with all known phenomenological results of polyelectrolyte dynamics, and some fresh predictions are made. © 1997 American Institute of Physics.

Notizen: Some important unique phenomena are encountered in gels of interpenetrating polymer networks (IPN) which arise from the coupling between phase transition, elasticity, and mutual entanglements and interaction of the underlying subnetworks. The relevant order parameters of IPN–gels are obtained as the local composition ratio fluctuations, the characteristic length of microphase separation and the volume fraction of the liquid. On the basis of a preaveraged Landau–Ginzburg theory, both microphase separation and volume phase transition are considered. There exists a common critical point at which the IPN–gel becomes critical for both types of phase transitions. © 1997 American Institute of Physics.

Notizen: Photoinduced Rydberg ionization (PIRI) spectroscopy has been applied to the problem of recording vibrationally resolved spectra of the dipole-forbidden B˜ 2E2g←X˜ 2E1g transition of C6H6+ and C6D6+. PIRI spectra of the B˜ state have been recorded via a number of vibrational states of the ionic ground state. A combination of Herzberg–Teller and pseudo-Jahn–Teller vibronic coupling between the B˜ state and a close lying C˜ state were used to explain the complex B˜ state vibrational structure. The two pseudo-Jahn–Teller vibrational modes, ν16 and ν17, were both found to be active in the PIRI spectra. In addition, evidence for a strong pseudo-Jahn–Teller interaction within each of these modes was found. The perturbed vibronic bands resulting from this pseudo-Jahn–Teller activity were successfully modeled using a two-mode pseudo-Jahn–Teller vibronic coupling model. Using a combination of the spectral results and the pseudo-Jahn–Teller calculations, we were able to generate absolute vibronic symmetry assignments for many of the lower B˜ state vibrational levels, from which vibrational assignments were made. The calculations also produced unperturbed vibrational frequencies for both pseudo-Jahn–Teller active modes along with values for their respective linear coupling constants. © 1997 American Institute of Physics.

Notizen: The rotational spectrum of the cyanomethyl radical CH2CN in its 2B1 ground electronic state was studied by microwave spectroscopy using a source-modulated spectrometer equipped with a free-space cell. TheCH2CN radical was generated in the cell by dc-glow discharge in pureCH3CN, and CD2CN inCD3CN. Fifty a-type R-branch rotational transitions were measured in the frequency region of 80 to 282 GHz, where fine structure and hyperfine structures due to the nitrogen and hydrogen nuclei were resolved for lower-N transitions, but only the fine structure for higher-N transitions. Similarly fine-structure resolved rotational transitions were measured for CD2CN in the frequency region of 190 to 286 GHz. The molecular constants of CH2CN, including the fine-structure constants and hyperfine coupling constants due to both the nuclei of N and H, were precisely determined by least-squares methods from 146 observed spectral lines, including 36 astronomically observed lines for the Ka=0components of N=1–0 and 2–1 from a dark cloud TMC-1 [Kaifu et al., Astrophys. J. Suppl. (submitted 1997)]. Those ofCD2CN were similarly observed from 92 observed spectral lines. The rotational constants of both the species were used to derive molecular structural parameters of CH2CN:r0(CN)=1.1919(13) Å, r0(CC)=1.3680(12) Å, andr0(CH)=1.0894(7) Å on the assumption of 120.22° for (angle)HCH. The spin densities of unpaired electrons estimated from the hyperfine coupling constants for the N and H atoms are consistent with the molecular structure obtained. © 1997 American Institute of Physics.

Notizen: The Fourier transform infrared spectrum of the in-plane CH3-rocking fundamental of 13CH3OH has been investigated at 0.002 cm−1 resolution. The rocking band is principally of parallel character and has a double-peaked Q branch and relatively wide spread subbands indicative of a substantial change in torsional barrier height. All A subbands from K=0 to 11 and all but one E subband from K=0 to 9 have been assigned in the n=0 torsional state and fitted to J(J+1) power-series expansions to obtain the subband origins and excited-state energy structure. The effects of vibrational interactions between the CH3-rocking and CO-stretching modes are prominent in the spectrum. Coriolis coupling between rocking (K−1) and CO-stretching K levels is observable for K≥6, and makes significant contributions to the subband origins and effective B values. Several J-localized perturbations due to level-crossing resonances with CO-stretch states have been observed and characterized. Two reported strong far-infrared laser lines optically pumped by the 10R(26) CO2 laser line have been found to arise through such a ΔK=3 level-crossing resonance. Modeling of the rocking-state torsion-K-rotation energies yields a height of V3r=469.2(38) cm−1 for the torsional potential barrier, a 26% increase over the ground state. The asymmetry K-doubling pattern in the excited state is qualitatively consistent with this barrier for K=2 to 4, but the K=5 rocking substate displays strongly enhanced splitting. © 1997 American Institute of Physics.

Notizen: It is shown that the newly completed accurate semiclassical theory for time-independent curve crossing problems can be usefully utilized to study various time-dependent curve crossing problems. Quadratic time-dependent problems can be solved exactly with use of the theory developed for the time-independent linear potential model. Furthermore, accurate and compact semiclassical theory can be formulated for general curved potentials. Even diabatically avoided crossing cases can be nicely treated. Multi-level problems can also be handled without difficulty with use of a new method to evaluate the necessary basic parameters directly from adiabatic potentials on the real axis in the fully diagonalized adiabatic representation. This method does not require a search for complex crossing points in the multi-level system, which is practically very difficult especially when the number of levels exceeds three. © 1997 American Institute of Physics.

Notizen: Gas phase ion processes of phosphine have been studied by theoretical calculations and experimental techniques. Ab initio quantum chemical calculations have been performed on the ion/molecule reactions starting from P+ in PH3, as they have been observed by ion trapping. P+ gives P2Hn+ (n=1,2) product ions with loss of H2 or H in different pathways and also reacts in charge-exchange processes to form PH3+. The energies of transition structures, reaction intermediates, and final products, as well as their geometrical structures have been determined by theoretical methods. The initial step is formation of a triplet P2H3+ adduct of C3v symmetry (P–PH3+). A hydrogen atom can either be directly lost from the tetracoordinated phosphorus, or first undergo a shift to the other P atom (HP+–PH2), followed by P–H bond dissociation. Dissociation of H2 from P2H3+ can also occur from both the initial P–PH3+ and HP+–PH2 species yielding PPH+. The heats of formation of the P2Hn+ ionic species have also been computed and compared with experimental data reported in the literature. © 1997 American Institute of Physics.

Notizen: We compute molecular Hartree-Fock-Kohn-Sham correlation potentials from ab initio coupled-cluster densities via a modified Zhao, Morrison and Parr [Phys. Rev. A, 50, (1994) 2138] scheme involving exact exchange. We examine the potential for several small systems, and observe complex structure. By fitting a functional expansion to our potentials we obtain a closed-shell functional which is an improvement over other pure correlation functionals in Hartree-Fock-Kohn-Sham calculations. The leading term in our functional is dependent on the number of electrons. Our results lead us to question the utility of correlation defined within the Hartree-Fock-Kohn-Sham scheme, and to consider alternative partitionings of the exchange-correlation energy.© 1997 American Institute of Physics.

Notizen: Vapor nucleation induced by seed ions of controlled size and charge is studied for several monovalent positive and negative ions as well as divalent cations. Ions from dissolved salts are transferred into a gas via electrospray, purified by differential mobility selection, and introduced into a condensation nucleus counter of the mixing type using supersaturated dibutyl phthalate vapors. The number of drops nucleated on the ions is then measured as a function of the vapor supersaturation S. None of the anions tested is activated. Singly charged cations with mobilities Z from 0.48 up to 0.93 cm2/V/s are activated at values of ln S 30% smaller than predicted by Thomson's model. All doubly charged cations tested (Z from 0.46 to 1.08 cm2/V/s) fall very near the Thomson curve, independently of their size or of whether the double ionic charge is localized on one metal atom or on two separate monovalent sites. © 1997 American Institute of Physics.

Notizen: The Mueller scattering matrix for sickle cell hemoglobin polymers is calculated using the coupled-dipole approximation method. The complex polarizability of the fiber is calculated using the absorption spectrum of hemoglobin to obtain the imaginary part and a Kramer–Kronig transform to obtain the real part. An anisotropy in the polarizability is calculated based on previous work using linear dichroism. The results of the polarized light scattering calculations are compared to previous measurements of total intensity light scattering and circular intensity differential scattering (CIDS) made on sickle red blood cells. Calculations of CIDS are found to be very sensitive to structural and optical parameters but reasonable agreement between experimental measurements and calculations are obtained. Further measurements and calculations should provide new structural information concerning the sickle fiber. © 1997 American Institute of Physics.

Notizen: Impingement of 16O-atoms on an 18O2 covered Pt(111) surface at 77 K induces the evolution of gas-phase 18O2 and 16O18O in a ratio of ∼4:1 independent of atomic flux or kinetic energy. The total initial probability of desorption of molecular oxygen induced by impingement of atomic oxygen is measured as a function of incident O-atom flux, kinetic energy, and angle. Interestingly, the probability is found to depend on incident angle with values ranging from 0.25±0.02 for a 60° angle of incidence to 0.42±0.02 for normal incidence. © 1997 American Institute of Physics.

Notizen: The pick-up technique has been employed to deposit single ammonia molecules as chromophores on the surface of large ArN clusters and to create small ammonia complexes attached to host clusters. Mass spectrometric investigations have been carried out to characterize the pressure dependence of the guest cluster formation. The technique is then combined with molecular beam depletion spectroscopy employing the infrared radiation of a pulsed CO2 laser. In the region between 920 and 1060 cm−1 in which the ν2 bending vibration (umbrella mode) can be excited, absorption spectra have been measured for the ammonia monomer as well as for the ammonia dimer, trimer, and tetramer adsorbed on large ArN host clusters. For the monomer a significant blue shift of 14.74 cm−1 is observed. Furthermore, it is found that the inversion motion associated with the umbrella mode is appreciably perturbed, resulting in a reduction of the inversion splitting by a factor of 0.3. As in the argon matrix, the rotation is only slightly hindered. The absorption spectra measured for ammonia dimers, trimers, and tetramers resemble very much the gas phase spectra of these complexes, except that they are blue shifted by 17 (dimer) and 6 cm−1 (trimer and tetramer). From this close resemblance it is concluded that the structures of the small ammonia complexes are not affected by the argon host cluster. © 1997 American Institute of Physics.

Notizen: Non-Hermitian complex effective Hamiltonians resulting from bound-continuous partitioning techniques are built from time dependent methods. We treat predissociation processes with a curve crossing. The energy dependent shift and half-width matrices are obtained simultaneously by a generalization of the wave packet Golden Rule treatment, as the real and imaginary parts of the Fourier transform of a memory kernel matrix. The latter contains auto- and cross-correlation functions. They are overlap integrals among the projections on the continuum of bound states multiplied by the interchannel coupling function responsible for the predissociation. These wave packets are propagated by the propagator of the sole continuous subspace. An approximate analytical expression of this correlation matrix is established for the harmonic/linear model. The numerical method is applied to the electronic predissociation of the MgCl A 2Π state, to a Morse/exponential model and to a predissociation with two coupled repulsive decay channels. The comparison between the correlation time scales and the Golden Rule lifetimes is decisive so as to justify whether the memory kernel can be considered as an impulsive kernel. This Markovian approximation implies that the two time scales are well separated. In the energy domain, this corresponds to the introduction of a mean phenomenological effective Hamiltonian that neglects the energy variation of the discrete-continuous coupling elements. We observe that the separation of the time scales is effective for weakly open systems, but not for overlapping metastable states for which the perturbative-theory widths largely exceed the mean energy spacing. This confirms from a temporal viewpoint that a nonperturbative treatment should not neglect the energy dependence of the effective Hamiltonian, as currently assumed in the study of largely open systems. © 1997 American Institute of Physics.

Notizen: In this paper we explore temporal vibrational coherence effects in nonadiabatic radiationless transitions between two electronic states in a large molecule or in the condensed phase, accounting explicitly for the role of the (intramolecular and/or medium) vibrational quasicontinuum of the final states. Our treatment of the time evolution of the wave packet of states and of coherence effects in the nonradiative population probabilities of the reactants and the products rests on the diagonalization of the Hamiltonian of the entire multimode system, with supplementary information being inferred from the effective Hamiltonian formalism. New features of the vibrational Franck–Condon quasicontinuum, which originate from weak, but finite, correlations between off-diagonal coupling terms, were established. The state dependence of the off-diagonal couplings Vsα between the doorway states manifold {|s〉} and the quasicontinuum {|α〉} was quantified by the correlation parameters ηss=〈VsαVαs〉/[〈Vsα2〉〈Vsα2〉]1/2, where 〈 〉 denotes the average over the relevant energy range. Calculations were conducted for a Franck–Condon four-mode system consisting of ns=100 doorway states and nα=3000 quasicontinuum states. The correlation parameters for all pairs of doorway states are considerably lower than unity (|ηss|(approximately-less-than)0.4), obeying propensity rules with the highest values of |ηss| corresponding to a single vibrational quantum difference, while for multimode changes between |s〉 and |s〉 very low values of |ηss| are established. Quantum beats in the population probabilities of products and reactants in nonadiabatic dynamics are characterized by an upper limit for their modulation amplitudes ξ≅(Γ/ΔE)η (for ΔE/2πΓ≥1), where Γ is the decay width of the doorway states and ΔE is their energetic spacing. These low ξ values originate from a small (∼Γ/ΔE) contribution to the off-diagonal matrix elements of the nonradiative decay matrix in conjunction with low correlation parameters. The amplitudes of the quantum beats in nonradiative temporal dynamics provide dynamic information on the larger correlation parameters ηss. Our theoretical and numerical analysis was applied for temporal coherence effects in nonadiabatic electron transfer dynamics in a Franck–Condon quasicontinuum of Mulliken charge transfer complexes [K. Wynne, G. Reid, and R. M. Hochstrasser, J. Chem. Phys. 105, 2287 (1996)]. This accounts for the "preparation" (signature of coherent excitation), for the low amplitudes of coherent temporal modulation of reactants and products (ξ≅0.05–0.06 determined by the ηss parameters) and for the dominating contributions to temporal coherence (subjected to propensity rules). © 1997 American Institute of Physics.

Notizen: Accurate, highly correlated calculations have been carried out for the ground electronic state and for a few of the lower excited electronic states, two of which are discussed in this work, of the trimer ionic helium cluster. Both linear asymmetric and nonlinear, symmetric, and asymmetric, configurations have been considered over a rather broad range of nuclear geometries. The results confirm the experimentally found [Chem. Phys. 102, 2773 (1995)] fragmentation patterns involving He and He+ fragments only and further suggest a range of possible nuclear geometries from which nonadiabatic couplings could also lead to He2+ fragments, albeit with a lower probability than the former channels. © 1997 American Institute of Physics.

Notizen: Monte Carlo simulations of the critical region of the restricted primitive model are reported. Using mixed-field finite size scaling analysis we show that the critical behavior is compatible with Ising like behavior although due to statistical error on the simulation data and large correction-to-scaling contributions mean-field behavior cannot be totally excluded. With the assumption of Ising criticality the critical temperature is estimated to be 0.0488±0.0002 and the critical density 0.080±0.005. © 1997 American Institute of Physics.

Notizen: A generalized eigenequation is derived for the optimal pairs of phase-unlocked pump-dump fields in the control of pure state molecular systems in the weak response regime. The associated eigenvalues are further correlated to the yields of pump-dump control, thus the globally optimal pair of phase-unlocked pump-dump fields is identified unambiguously. Presented is also a hierarchy of reduction from the general Liouville-space density matrix control formulation in the strong response regime to various linearized control equations in the weak response regime. Application to the control of a molecular ro-vibronic level in the ground electronic surface reveals a novel symmetry relation between the optimal pump field and its counterpart of the optimal dump field. © 1997 American Institute of Physics.

Notizen: Reactions of small thermalized positively charged nickel clusters with carbon monoxide were studied in a molecular beam experiment. The nickel clusters were produced in a high intensity cluster ion source and thermalized in a large helium-filled quadrupole ion guide. The clusters were size selected by a quadrupole mass spectrometer. The mass- and charge-selected nickel clusters then passed through a linear quadrupole drift tube filled with a mixture of helium buffer gas and carbon monoxide. The reaction products were then analyzed by a quadrupole mass-spectrometer. Using this technique, saturation limits for Nin+ clusters with n=4–31 were measured and the competitive reaction channels were identified. Under certain experimental conditions carbide formation was observed in the case of the nickel tetramer, pentamer, and hexamer. The structure of the nickel carbonyl clusters is discussed within the framework of the polyhedral skeletal electron pair theory. The cluster growth may be explained by a pentagonal sequence of structures for n=4–7, capping of the pentagonal bipyramid to buildup an icosahedron at Ni13+, and further capping of this icosahedron to form a double icosahedron at Ni19+. © 1997 American Institute of Physics.