Molecular beam scattering experiments have emerged in recent years as an important tool for investigating structural disorder on surfaces. This paper gives a brief overview of the developing theory of scattering from disordered surfaces, which is essential for the interpretation of such experiments, and presents new results on this topic. A brief discussion is given of the methods developed for calculating atom scattering from disordered. systems (e.g. time-dependent quantum wavepackets; distorted-wave approximation; sudden approximation) and of simulations of disordered surface structures (by Monte Carlo or molecular dynamics). The paper focuses, however, on features of the scattering intensities and their relation to disordered surface structures. The main topics include: (1) The concept of cross section for He scattering by an isolated defect on a surface, and its application to the study of interactions between defects. Comsa and his coworkers developed this into a most powerful tool for studying surface defects and defect interactions. We examine the assumptions involved, and the information contained in the measured cross section on defect shape and geometry, and on He/defect potentials. (2) Angular intensity distribution for He scattering from isolated defects; defect rainbow and Fraunhofer effects. (3) He scattering from periodic, substitutionally disordered surfaces. New results are presented on He scattering from mixed Xe + Kr monolayers on Pt(111), comparing theory with the experiments of Comsa et al. The results indicate that these monolayers are indeed periodic and (almost) perfectly substitutionally disordered for all Xe:Kr ratios. Rainbow intensity features due to the substitutional disorder are calculated and analyzed. (4) He scattering from amorphous mixed monolayers, with application to Xe + Ar mixtures on Pt (111). (5) Atom scattering from liquid surfaces, e.g. structure and dynamics of liquid Hg probed by Ar scattering. The paper concludes by emphasizing some of the outstanding open problems in the field.

The yield for photodissociation of Cl2 in solid Ar was measured experimentally as a function of photon energy and studied by molecular dynamics simulations. The results show that separation of the Cl fragments at low energies occurs by delayed exit (t greater than or similar to 3 ps) from the surrounding cage of Ar atoms. Above a photon threshold energy of greater than or similar to 9.0 eV, there is a switchover to direct cage exit, in which the Cl impulsively knocks a cage atom out of its way. The results are a first demonstration of delayed cage exit and of changeover from delayed to direct exit in solid-state photolysis.

A new method is proposed for dealing with difficulties in molecular dynamics (MD) simulations caused by nonpreservation of zero-point energies (ZPE) in classical dynamics. Specifically addressed is a difficulty, for molecules held in weakly bound clusters, of energy flow from the initial ZPE of stiff molecular vibrations into soft cluster modes, causing unphysical dissociation or melting of the cluster. The remedy proposed is a classicallike MD algorithm, which treats the stiff modes by semiclassical Gaussian wave packets and the soft modes by classical dynamics, using the time-dependent self-consistent field (TDSCF) approach to couple the classical and the semiclassical modes. The resulting algorithm is very similar in form to classical MD, is computationally simple, stable, and appears free of unphysical effects. The method is illustrated by test applications to models of the clusters I2He and (HBr)2 in the ground states, which dissociate at the expense of their ZPE classically, but remain stable in the new method.

The photolysis of HCl in the cluster Ar ... HCl is studied theoretically with the objective of elaborating on the effect of a single ``solvent'' atom on the dynamics of chemical bond breaking. The focus is on observable properties, such as the velocity distribution and the angular distribution of the H atom product, on how these properties reflect the ``solvent'' effect, and on the physical mechanisms involved. The main results obtained are the following. (1) There is a high probability for at least a single ``hard'' collision,between the H photofragment and the Ar atom before the H atom leaves the cluster. Multiple collisions between the H and the heavy atoms also occur with significant probability. (2) The final kinetic-energy distribution of the H atom shows a long pronounced tail due to energy transfer in the collisions with the heavy atoms. (3) There are pronounced peaks in the angular distribution of the H atom due to the single and multiple collision events. (4) Comparison of photolysis of Ar ... HCl with that of Ar ... DCl shows a large isotope effect, again due to collisions within the cluster during its fragmentation. The results were mostly obtained from classical trajectory calculations, but also in part from calculations using a hybrid quantum/classical method in which the H atom is treated by quantum wave packets while the heavy atoms are described classically. The quantitative results show some quantum effects, but for most purposes the classical description is sustained. Implications of the results for experimental studies of molecular photodissociation in clusters are discussed.

The photodissociation of HCl in the cluster Ar...HCl by an extremely short pulse was studied using a hybrid quantum mechanical/classical approach. In this method, the H atom is treated quantum mechanically, the heavy atoms classically, and the time-dependent self-consistent-field (TDSCF) approximation is used to couple the quantum with the classical modes. The results are compared with those of classical trajectory calculations. On the whole, good qualitative agreement is found between the results of the (partly quantum) hybrid method and the pure classical ones. However, quantum interference effects of quantitative significance are found both in the angular and in the kinetic energy distribution of the H atom product. These effects, and resonances that contribute to the process, are analyzed in terms of wave packets obtained for the H atom in the hybrid method. The usefulness and applicability of the hybrid method are discussed in the light of the results.

Theoretical and experimental results are compared for the 257 nm photolysis of methyl iodide adsorbed on an MgO(100) crystal. Molecular-dynamics calculations treat CH3I as a pseudodiatomic molecule and describe the geometry and the vibrational and librational frequencies of ground State CH3I on the surface of a solid at 125 K. The simulations modeled the photodissociation dynamics of the adsorbed species. The photoexcitation of CH3I at 257 nm is to the 3Q0 state which is, in tum, coupled to the 1Q1 state. The electronic surface coupling allows for two dissociation pathways, producing either ground- or excited-state iodine atoms in concert with ground-state methyl radicals. The I*/I branching ratio and the velocity and angular distributions of both photofragments are predicted by the theory. A comparison is made between these predictions and experimental observation of the I*/I branching ratio, the velocity distribution of the methyl fragment, and the internal state distribution of the methyl. A substantial lowering of the I*/I ratio as compared to data from the gas-phase photodissociation studies is both predicted by theory and seen experimentally. Theoretical simulations attribute this change to efficient trapping of the I* photofragments by the surface. Further comparisons between the theoretical predictions and the experimental data support a model where the molecule is aligned perpendicular to the surface and the escape of iodine atoms from the surface following the photodissociation of adsorbed methyl iodide involves collisions with the surface.

An overview is given of a recent method for dynamical simulations of many-atom systems, which incorporates also important quantum effects. The method treats a few pertinent degrees of freedom (e.g., a light atom) by time-dependent wavepackets and all the other coordinates by classical trajectories. The classical and quantum subsystems are coupled in this approach by a hybrid quantum/classical Time-Dependent Self-Consistent Field (TDSCF) approximation. The properties, validity, range, and limitations of this method are discussed, and numerical tests of its accuracy are presented for simple model systems. The method is illustrated by applications to photodissociation of HI molecules in solid Xe and in Xe clusters of different sizes. The scope of potential applications, open problems, and possible directions of extending the method are discussed.

Molecular-dynamics simulations were used to calculate the photodissociation yield of F2 in solid Kr as a function of photon energy at 12 K. The calculations used pairwise interactions including empirical gas-phase F(2P3/2)/Kr potentials. Excellent agreement was found with recent experimental data, showing the quantitative accuracy of molecular-dynamics simulations using pairwise reagent/solvent potentials, in describing a chemical condensed-matter reaction.

The photodissociation of HCl in the Ar-HCl cluster has been studied theoretically using a classical trajectory method to explore the effect of a ``solvent'' atom on the process. The calculated kinetic energy and angular distributions of the photofragments show a ``cage'' effect, due to collisions between H and the heavy atoms before H leaves the cluster. Also, significant differences are seen depending on whether a classical or a quasiclassical distribution is used to describe the initial Ar-HCl bending distribution.

He scattering from substitutionally disordered Xe + Kr monolayers is studied theoretically. The angular distribution of the scattered He, and lifetimes of scattering resonances at the surface, are calculated for different Xe:Kr mixing ratios. In addition to Bragg diffraction, new intensity maxima are found. These are interpreted as rainbow effects (disorder rainbows), and may be very useful in characterizing disordered surfaces. Also, trapping resonances are found, which are sharply sensitive to the percolation thresholds for the mixtures. The results show He scattering can be a powerful tool in studying 2D percolation behavior.

A new method is presented for calculating the quantum dynamics of processes in polyatomic systems. The approach uses the time-dependent self-consistent-field (TDSCF) approximation, which treats each mode as moving in a time-dependent mean field. The new method corrects the TDSCF by a perturbation-theoretic algorithm. The method is tested against numerically exact calculations for vibrationally inelastic He + H-2 collisions, and for reactive H + H-2 scattering. Very good agreement was found with the exact results. Major improvement was obtained over (uncorrected) TDSCF. The method is computationally efficient, practically applicable to systems of many degrees of freedom.

Vibrational energies and eigenfunctions of Ar3, including some pertaining to highly excited states, are computed, and insights into their dynamical and structural properties are obtained. The method used employs the vibrational self-consistent-field (SCF) theory in hyperspherical coordinates as a first approximation. Exact results are obtained by configuration interaction, using the SCF states as an efficient basis. A focal point of the study is the effect of three-body potentials on the vibrational spectrum. Axilrod-Teller and other three-body potentials are used to examine this. It is found that the effect of three-body forces on the spectrum is substantial, and larger than effects due to uncertainties in the presently known two-body Ar-Ar potentials. This suggests that experimental spectroscopy of Ar3 may be used to determine reliable three-body forces among Ar atoms. It is also shown that the three-body double-dipole-quadrupole interaction, while less important than the Axilrod-Teller one, has a significant effect on the vibrational spectrum. Finally, a detailed analysis is made of the Ar-Ar distance distributions in the various states, of the structural distributions of Ar3, and of the properties of the wave functions. We find that the wave functions show well-ordered nodal patterns even for the highly excited large-amplitude states. Thus, these states do not correspond qualitatively to ``liquid-like'' behavior of the cluster.