The time-dependent self-consistent-field (TDSCF) approximation is used to study the photodissociation of the Ar-HCl cluster in a three-dimensional framework. The results are compared with numerically exact quantum calculations, and the properties and accuracy of the TDSCF approach are evaluated on this basis. The TDSCF approximation is used in Jacobi coordinates, and the total wave function is factorized into a wave packet for two coordinates associated with the H atom, and a wave packet for a single coordinate that describes the relative motion of the heavy particles. Quantitative agreement between the TDSCF and the exact results is found for most quantities calculated. The calculations show that photodissociation, and in particular the departure of the H atom is predominantly a direct process, but an appreciable amount of wave packet amplitude moving in excited state resonances is also found. This amplitude seems significantly larger than obtained in recent calculations by Schroder et al. [J. Chem. Phys. 100, 7239 (1994); Chem. Phys. Lett. 235, 316 (1995)]. The validity and computational efficiency of the TDSCF approach for realistic systems of this type is discussed. (C) 1995 American Institute of Physics.

The harmonic approximation for the potential energy of proteins is known to be inadequate for the calculation of many protein properties. To study the effect of anharmonic terms on protein vibrations, the anharmonic wave functions for the ground state and low-lying excited states of the bovine pancreatic trypsin inhibitor (BPTI) were calculated. The results suggest that anharmonic treatments are essential for protein vibrational spectroscopy. The calculation uses the Vibrational self-consistent field approximation, which includes anharmonicity and interaction among modes in a mean-field sense. Properties obtained include the quantum coordinate fluctuations, zero-point energies, and the vibrational absorption spectrum.

The photodissociation of HCl on MgO(001) is studied by classical and quantum methods. The quantum aspect resulting from the hydrogen zero-point motion is also modeled in the classical simulation and has an important influence on the dynamics through the initial distribution of tilt angles from the surface normal, The angular distribution of the scattered photofragments and the kinetic energy release of hydrogen show characteristic structures due to rainbows, scattering shadow and resonances. Information about surface potential and adsorbate geometry can be obtained from them.

The time-dependent self-consistent field (TDSCF) approximation is applied to the coupled-mode vibrational dynamics of highly anharmonic quantum clusters, and its validity and accuracy for such systems are tested against exact quantum calculations. The calculations are pursued for bound but non-stationary states of collinear models of (Ne)(3) and B(H-2)(2). It is found that for a short timescale (t less than or equal to 1 ps) the TDSCF approximation is in excellent accord with the exact results. This suggests that TDSCF should be an attractive quantitative approach for the short timescale vibrational dynamics of highly anharmonic quantum clusters.

Molecular dynamics (MD) computer simulations are carried out for scattering of high-energy Xe atoms off liquid squalane, and the results are compared with those of molecular-beam scattering. experiments. A crude model for squalane is adopted, describing the hydrocarbon chain molecule as a sphere, and ignoring the role of internal modes. Good overall agreement is found between the results of the simulations and experiment, both for angular distributions and for trends in energy I transfer properties. In particular, excellent agreement is obtained for the dependence of the energy transfer on the deflection angle for in-plane scattering. Theory predicts less trapping events than found experimentally, probably due to the crude model adopted for the squalane molecules. The partial success of the model in predicting some properties and not others is discussed. The other main conclusions of the study are (1) The instantaneous local structure of the liquid surface is highly corrugated, giving rise to a broad annular distribution and to extensive out-of-plane scattering. (2) High-energy atoms undergo both a trapping desorption and also direct inelastic scattering, the latter. yielding information on liquid structure, (3) The angular distribution of atoms at a selected final velocity is sensitive to the local structure and dynamics of the surface. (4) The direct scattering can be conveniently interpreted in terms of contributions from single, double, and multiple collision events, these being roughly equal in relative weight. Forward scattering at grazing angle is dominated by single collisions, while double and multiple collisions have higher contribution at other directions. The double collision contribution in particular contains structural information. (5) There is a substantial yield per collision for sputtering of the squalane-like soft spheres. These results provide insight into the dynamics of gas-liquid collisions, and indicate the usefulness of beam scattering asa tool for studying liquid structure and dynamics.

A method for calculating decay rates of vibrational modes in large polyatomic systems is proposed and tested. The high frequency excited vibration is treated quantum mechanically, and the soft modes are described classically. The initial state is described by the hybrid quantum/classical self-consistent-field (SCF) approximation. The formalism is based-on a golden-rule expression. The driving potential is the difference between the full Hamiltonian and the mean field Hamiltonian (SCF) causing the decay of the initial state to final mixed quantum/classical ;SCF states. These states are calculated using an extension of the usual static mean-field techniques to systems with mixed quantum and classical degrees of freedom. The formalism for obtaining the mean-field states and calculating the decay rates is presented, and the method is applied to a diatomic molecule treated quantum mechanically, embedded in a 1D model for a. rare gas cluster treated classically. The dependence of the eigenenergies of the quantum and the decay rates with temperature is studied. The influence on the system size is also presented and compared with the prediction of the isolated binary collision model. The effect of a change in the linear density of the cluster on the eigenenergies of the vibrational mode is presented.

Photodissociation of ICN by W excitation in solid and liquid Ar is studied by molecular dynamics simulations. The focus is on the differences between the cage effects on the CN photoproduct in the two phases, and on the excited state isomerization ICN* –> INC* dynamics in the solid matrix. Nonadiabatic transitions are neglected in this first study. The main results are: (1) No cage exit of the CN product is found in solid Ar, even in simulations at temperatures close to melting and for large excess energies. The result is in accord with recent experiments by Fraenkel and Haas. This should be contrasted with the large cage-exit probabilities found in many systems for atomic photofragments. The result is interpreted in terms of geometric and energy transfer considerations. It is predicted that complete caging of diatomic and larger photofragments will be typically the case for photodissociation in rare-gas matrices. (2) Almost 100% cage-exit probability for the CN product is found for ICN photolysis on the (II1)-I-1 potential surface in liquid Ar. On the other hand, photolysis on (II0+)-I-3 potential surface does not lead to cage exit on a time scale of 15 ps. The large differences between the reaction in the solid and in the liquid, and between the behavior of the process on the (II0+)-I-3 and the (II1)-I-1 potentials, respectively in the liquid, are interpreted. (3) CN rotational dynamics and subsequent relaxation leads to isomerization int he excited electronic states. On the (II0+)-I-3 potential surface one finds after t greater than or similar to 0.5 ps roughly equal amounts of the ICN and INC isomers. On the (II1)-I-1 surfaces only INC is found after t greater than or similar to 3.5 ps. This is explained in terms of the barriers for CN rotation in the two excited states, and in terms of time scales for rotational relaxation. The results throw light on the differences between cage effects for photochemical reactions in solid and in liquid solution, and on cage-induced isomerization dynamics in solid matrices.

The photodissociation dynamics of HCl in the clusters Ar2-HCl and Ar-HCl is studied in order to explore the cluster size effect. A quasi classical trajectory approach is employ to simulate the photofragmentation dynamics. interesting manifestations of the cage effect are found in the light and the heavy atom kinetic energy distributions, with important differences between the two clusters. The Ar and Cl distributions provide separate information on dynamical events which cannot be resolved in the hydrogen distribution. Only two different excitation wavelengths are used in the simulations. The cage effect is found to be strongly dependent on the wavelength employed to excite the HCl molecule. This is so to the extent that the trend of an increasing cage effect with the cluster size is reversed for the smaller wavelength. New types of resonance behavior are observed for the hydrogen motion in Ar2-HCl compared with the Ar-HCl cluster. An interesting difference between Ar-HCl and Ar2-HCl is that, in the latter case, Ar2 can form as product of the photodissociation, with a high yield for the two wavelengths. The dynamics of the process and the implications of the results are discussed.

A general method for studying transition state spectroscopy and dynamics in hydrogen atom transfer reactions is presented. This approach is based on the time-dependent self-consistent field (TDSCF) approximation and is applied to a study of the CIHCl- photodetachment experiments of Metz et al. [Metz et al., J. Chem. Phys. 88, 1463 (1988)]. Comparison of results of exact time-dependent and TDSCF calculations are made for collinear and three-dimensional (J=0) approximations for the quantum dynamics. When ClHCl is constrained to be collinear, the TDSCF calculation overcorrelates the motions in the R atom displacement and ClCl extension coordinates. This results in relatively poor agreement with the exact result for many properties of the wavefunction. in contrast, when the system is propagated in the three vibrational coordinates of the system, the transition state dynamics are effectively overmuch more rapidly. Consequently, the TDSCF approximation yields results of very good quantitative accuracy over the time required for most of the wave function to decay off of the transition state. Comparison is also made between the wave function that results from the exact propagation and from TDSCF when the wave function in the ClCl stretch coordinate is approximated by a Gaussian wave packet. Here the magnitude of the overlap between the two TDSCF wave functions in the H atom coordinates, for quantum and semiclassical propagations of the wave function in the ClCl distance Coordinate, is greater than 0.98 over the time of the propagations. These TDSCF calculations are repeated fbi a wave function that is approximated by a product of a two-dimensional wave function in the hydrogen atom coordinates and a one-dimensional wave function in the ClCl extension coordinate and even better quantitative agreement with the exact propagation is achieved. The success of this method for studying ClHCl gives us confidence that TDSCF will provide a general powerful tool for studies of hydrogen and proton transfer reactions in large systems.

The structure and stability of clusters of a boron atom with one to eight H-2 molecules is investigated. For the simplest BH2 clusters, the lowest ab initio adiabatic potentials for o-H-2 and p-H-2 interacting with a boron atom are used. For the larger clusters (n=2-8), the p-Hz is treated as a sphere, and the total potential is taken to be the sum of pairwise additive B-H-2 and H-2-H-2 interactions which include, in the former case, an anisotropy due to the orientation of the unpaired B 2p electron. This electronic interaction is considerably more attractive when H-2 approaches the B atom in a plane perpendicular to the orientation of the 2p orbital. The local and global minima on these potential surfaces were located and diffusion quantum Monte Carlo simulations were used to determine the energies and properties of the ground state wave functions for these B-(H-2)(n) clusters. For the B(H-2) cluster, a comparison is made with the results of variational calculations.

The formation of HCl molecules in collisions of H atoms with Cl(Ar)12 Clusters is studied by classical trajectory calculations as a function of the cluster temperature. At sufficiently low impact energy (E = 10 kJ/mol) a pronounced threshold effect with temperature is observed: In the solidlike regime of the cluster (T less-than-or-equal-to 40 K) there is strong screening of the Cl atom by the icosahedral Ar solvation shell preventing H+Cl association. In the liquidlike phase of the aggregate (T almost-equal-to 45 K) a sharp onset of reactivity is found with a reactive cross section of 11.4 x 10(-20) m2.