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.

We discuss a generalized dynamic mean-field method combining the advantages of explicit pair correlations and of configuration interaction. The approximate dynamical method, which we call time-dependent self-consistent-field configuration interaction (TDSCF2-CI), is constructed by including N(N-1)/2 TDSCF2 configurations. In each configuration a given pair of N coupled modes is directly (not in the mean-field approach) correlated; the N(N-1)/2 configurations include all such choices of pairs. As such, it has both the usual advantages of TDSCF and improvements due to some inclusion of correlations (exact results for any two-mode problem, improved descriptions of dynamical corrections, and greater accuracy). A three-mode model Hamiltonian is analyzed using five approximate treatments, i.e., the usual TDSCF, the three TDSCF2 forms, and the TDSCF2-CI one. The quantities for comparison with the exact results include the decay P(t) of the initial state, the time dependencies of the energies e (i) of individual modes, and the overlap S (t) of the corresponding approximate wave function with the exact one. We find, indeed, that explicit inclusion of pair correlations improves the description of the quantum dynamics of the system.

A theoretical investigation of the photodissociation dynamics of (HCI)2 at 193 nm is presented. Prior to excitation, the cluster is taken to be in its rotation-vibration ground state. A quantal description of this six-dimensional wave function is computed using diffusion quantum Monte Carlo (DQMC). The photodissociation dynamics are simulated by classical trajectories in which the molecule undergoes vertical excitation to an electronic state that is repulsive along one of the HCI stretch coordinates. The initial conditions for these trajectories are sampled according to the Wigner function which was obtained from the DQMC wave function. In a significant fraction of these trajectories, there is a reactive collision in which the H atom interacts with the H’Cl’ molecule to form HCI’. Of the remaining collisions, most are nonreactive, but a small number lead to H-2 formation. The trajectories in which an exchange reaction occurs result typically in formation of HCI’ molecules that are rotationally and vibrationally hotter and in H atoms with lower kinetic energies than are found in the nonreactive trajectories. Resonances, in which the H atom undergoes multiple collisions with Cl and H’Cl’, are observed in all three classes of trajectories. The above results indicate that this is a rich system for the study of photoinduced chemical reactivity in hydrogen-bonded clusters.

The reaction between an O(3P) atom and a hydrocarbon molecule weakly bound to an argon atom was studied by classical trajectory simulations. The results are compared to those obtained for the reaction of a free hydrocarbon. A simplistic model system was constructed in which the hydrocarbon was represented as a pseudodiatomic molecule. Although simple, the model reproduced correctly the internal energy distribution in the OH produced in the reaction of the free species. It was found that the OH, produced from the reaction of the van der Waals complex, emerges with less internal energy and less translational energy than the OH from the monomeric process. In the case of the complexed reagents, the collision complex lifetime is longer and the oxygen explores portions of the potential energy surface that are not available in the monomeric reaction.

A theoretical study is presented on the photodissociation dynamics of Cl2 in crystalline xenon at 100 K, and within a range of pressure between 0 and 100 kbar. Temperature/pressure ensemble molecular dynamics simulations were carried out. The potentials used were accurate enough to reproduce the experimental equation of state of solid xenon. The results show that the photodissociation quantum yield varies strongly with pressure, falling from 30% at zero pressure, to 2% at 12.5 kbar, and 0% at higher pressures. These yields are in good agreement with experimental measurements. This behavior is found to be due to the strong effect of pressure on the librational (rotational) amplitudes of the Cl2 molecule and to the sharp dependence of the photodissociation yield on the molecular orientation in the reagent cage.

Evidence for a cage effect in the 193 nm photodissociation of HBr in the Ar-HBr cluster is found. This effect manifests itself as a tail extending toward lower energies in the hydrogen photofragment kinetic energy distribution (KED). This is a consequence of energy transfer in collisions between the light and the heavy atoms. There is good agreement between the experimental and theoretical KEDs.

The diffraction of thermal He atoms from mixed Xe + Kr monolayers on Pt(111) was measured, and the results were compared with theoretical studies of these systems. The results shed light on the structural properties of these disordered systems, and on their relation to the He diffraction intensities. Experimentally, the specular (0,0), the (1,0), and the (2,0) Bragg peak intensities were measured for monolayers of different Kr:Xe concentration ratios. The theoretical calculations included Monte Carlo simulations of the mixed disordered monolayers, and quantum calculations in the Sudden approximation of the scattering intensities from the simulated disordered structures. The following main results were obtained: (1) Both experiment and the Monte Carlo simulations suggest that the mixed Xe + Kr monolayers are periodic for all Xe:Kr concentration ratios, the lattice constant varies linearly with the Xe:Kr ratio. The domain size of the 2D crystals, from experiment and theory, is found to be larger than 100 angstrom. (2) The Monte Carlo simulations suggest that the Xe + Kr monolayers form an almost ideal substitutionally disordered lattice. (3) Using a semiempirical Debye-Waller factor, reasonable agreement is found between the theoretical and the measured diffraction intensities, thus supporting the calculated structural model for the disordered surface. (4) The theoretical scattering calculations show that in addition to the diffraction peaks, there are also intensity maxima at non-Bragg positions. These are entirely due to the lattice disorder, and are identified as a recently found new type of Rainbow effect that can furnish important information on disordered surfaces. The results demonstrate the power of He scattering as a tool for exploring substitutionally disordered surfaces.

A hybrid quantum/semiclassical method is proposed and applied to study realistically the dynamics of the three-fragment photodissociation process Ar ... HCl + hv –> Ar + H + Cl. In the method the hydrogen motion is treated by exact quantum mechanics, while the heavy atoms are described by semiclassical Gaussian wave packets. This treatment is expected to reproduce the main quantum features of the dynamics. Part of the wave packet is found to describe resonance events in which the light particle is temporarily trapped inside the Ar ... Cl cage and oscillates periodically between the heavy atoms before it dissociates. Interference between frequency components of the H wave function that populate different resonance levels give rise to interesting quantum effects. Such effects appear in the angular distribution of the hydrogen fragment, which shows some diffraction oscillations, and scattering into classically forbidden regions. Quantum interferences between the resonances are also the cause of a pronounced structure of peaks in the H photofragment kinetic energy distribution (KED). Time-correlation functions of the wave functions involved are computed, and the implications for the absorption spectrum and its relation to the KED of the H atom are discussed. The results demonstrate the power and applicability of quantum/semiclassical time-dependent self-consistent-field (TDSCF) as a tool for studying the dynamics and spectroscopy of realistic molecular systems.

The close coupling wave packet (CCWP) and quasiclassical trajectory methods are used to study rotationally inelastic scattering of N2 from static, corrugated surfaces. The collision energy in these calculations ranges from 10 to 100 meV; 18 711 quantum states are included in the highest energy calculations to ensure convergence. The scattered molecules are analyzed with respect to the polarization of the final angular momentum vector and the amount of energy transferred into rotational motion and translational motion parallel to the surface. Comparisons of quantum and quasiclassical results show that quantum effects are important even with the relatively large mass of N2 and the high scattering energies used and can be seen even after summing over many final quantum states. A test of a factorization relation derived from the coordinate-representation sudden (CRS) approximation gives qualitative agreement with the exact quantum results.

A hybrid quantum/semiclassical simulation method is applied to the photodissociation dynamics of HCI in the Ar-HCI cluster. The method treats the hydrogen quantum mechanically, and the heavy atoms by semiclassical wavepackets. The dynamics of the H atoms if found to show resonances where the H atom rattles between the Ar and Cl atoms before leaving. Particularly interesting is the kinetic energy distribution of the H atom which shows a structure of pronounced peaks, associated with the resonance levels, while the absorption spectrum is structureless. The dynamics of the process is discussed.

The photodissociation of HCl in the Ar-HCl cluster is studied theoretically, with the focus on the angular distribution of the H atom photofragments. Excited state resonances, in which the H atom rattles between the heavy atoms, contribute to the process. It is found that for excitation into a resonance state, the measurable angular distribution of H atoms from Ar-HCl clusters oriented in space provides a mapping of the resonance wave function. This predicts the possibility of imaging resonance wave functions in such processes.

A semiclassical time-dependent self-consistent-field (TDSCF) method is developed for dealing with the difficulties caused by the nonpreservation of zero-point energy in classical molecular dynamics simulation. The method is applied to a collinear model of a (Ne)12 cluster at very low temperatures. Classically, this system dissociates rapidly due to its zero-point energy. We show that the system remains stable when treated by the new method. The normal mode dynamics of the anharmonic cluster are calculated and discussed. Interesting results are obtained on the lifetimes of single-mode states and energies due to the coupling between the modes. Some of the single-mode states have subpicosecond lifetimes, while others are stable for at least 60 ps. The results illustrate the power of semiclassical TDSCF as a tool for studying the vibrational dynamics of anharmonic cluster at very low temperatures.

The van der Waals cluster molecule ArCO2 is studied computationally by using the vibrational self-consistent field (SCF) approximation, with an approximate but reasonable potential function. Calculations are carried out both for the full six-dimensional motion and for a reduced two-dimensional problem in which the CO2 is held rigid. An interesting dynamical transition is found in the motion of the Ar atom. Its equilibrium geometry is a symmetric T-shape, and for low excitations both the radial and the angular motions in the CO2 plane resemble the states of anharmonic oscillators (smaller intervals with higher excitations). Above the sixth state of the bend in the angle theta, however, the bend spectrum changes to that of a rigid rotor, with spacings of 2Bn(theta) for quantum number n(theta). The one-dimensional effective SCF potentials along the theta coordinate and plots of the wave function both show a dynamical transition, in which, above n(theta) = 6, the motion of the Ar in the CO2 plane is essentially that of a rigid rotor in the theta coordinate. Calculations of the principal moments of inertia support this interpretation.