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Notes: We used a randomly corrugated hard wall model and the sudden approximation to analyze two experiments on atom scattering from disordered surfaces. In one, the structural surface disorder was caused by ion bombardment. In the other, the disorder was due to an incomplete overlayer of adsorbed atoms. We also present a study of the scattering of a rigid rotor by a randomly corrugated hard wall using the sudden approximation.

Notes: The photodissociation dynamics of a collinear model of the van der Waals cluster Xe–HI is used as a testing ground for time-dependent self-consistent field (TDSCF) approximations. In this study, the quantum-mechanical TDSCF and a combined classical/quantal TDSCF (in which the light atom is treated quantum mechanically, the heavy atoms are treated classically) are compared to numerically exact wave packet calculations. Very good agreement is found between the TDSCF approximations and the exact result over the entire subpicosecond time duration of the process. In particular, all the properties related to the quantal degree of freedom in the combined quantal/classical TDSCF method reproduce almost perfectly the exact results. However, the classical mode in the hybrid approximation is somewhat less well described due to insufficient representation of energy transfer between the modes. The conclusions are very promising as to the applicability of TDSCF methods, in particular the hybrid quantal/classical scheme to more complex systems in which only a few degrees of freedom can be treated quantum mechanically.

Notes: The photodissociation of isolated Cl2 impurities in a Xe crystal was investigated by molecular dynamics simulations. The calculations were carried out for a photodissociation energy of 3.775 eV, and for several temperatures in the range from 10 to 150 K (the melting point is 162 K). The focus was on the physical mechanisms whereby the product atoms exit from the cage, on the properties of the final sites occupied by the Cl atom, and on the temperature dependence of the processs. The main findings were: (1) exit of a Cl atom from the original reagent cage, when it occurs, is always direct upon photodissociation, and does not involve multiple collisions with the surrounding cage walls. This is in qualitative contrast with the dynamics of cage exit in the case of HI photodissociation in Xe at very low temperatures, found in a previous study. (2) The occurrence of product exit from the cage depends entirely on whether the reagent molecule has been oriented at the direction of a transition state for the exit at the instant of photodissociation. (3) The temperature threshold of Cl exit from the cage is 95 K, and essentially coincides with onset of free rotation for the reagent molecules in the host crystal. (4) The temperature dependence of the probability for cage exit is strongly nonmonotonic: The probability increases as T increases from 95 to 110 K, falls off to 0 around 125 K, then increases again as T approaches melting. (5) At the photodissociation energy used, the only site that the Cl atoms occupy in the new cage is the octahedral interstitial site. Various aspects of reaction dynamics in crystalline solids are discussed in the light of the above results and by their comparison with findings of a previous study on photodissociation of HI in Xe.

Notes: The photodissociation of HI impurities in a crystalline Xe host is studied by molecular dynamics simulations. From the calculated trajectories, analyses are given for: The behavior of the HI molecule before dissociation, the motions of the fragments following photon absorption, and the sites and vibration dynamics of the H fragments long after dissociation. The main findings include: (i) The photodissociation yield as a function of temperature is not monotonic (0 at 0 K, 0.2 at 17 K, 0.1 at 35 K). (ii) The nascent H atoms, at early time (∼0.5 ps), exhibit well defined, high frequency (∼900 cm−1) vibrations in the cage. The H trajectories acquire increasingly a random walk character with the progress of time. (iii) H exit from the cage is virtually never direct. (iv) After relaxation to equilibrium, the product H atoms occupy dilated interstitial sites (nearest-neighbor xenons displace by 0.3 and 0.6 A(ring) at the octahedral and tetrahedral interstitial sites, respectively). (v) The H atom dynamically distorts the octahedral site and exhibits three well-separated local vibrational frequencies, corresponding to motions along well-defined axes of the site. (vi) The reagent HI molecular rotations are strongly hindered at low temperatures, and are more aptly described as large amplitude bendings associated with the complex Xe⋅⋅⋅HI. The experimental implications of the above findings and the possible consequences of quantum effects are discussed.

Notes: A study has been made of the vibrational energy flow mechanisms and time scales pertaining to the overtone stretch excitations of methyl and acetylenic CH stretches in propyne. Classical trajectories are used to interpret the experimental data for the overtone linewidths, as well as to analyze the role that individual modes play in determining energy flow. The full anharmonic potential surface for these calculations, including all modes, has been developed from spectroscopic and structural information, including the linewidth data. The principal results are: (1) The trajectory calculations show a localization transition, corresponding to a switch over from normal-mode behavior for CH3 excitations up to v≅3 to a local-mode CH excitation within the CH3 moiety for excitations of v(approximately-greater-than)6, with transition behavior for v=4,5. (2) The acetylenic CH shows local-mode behavior from v=1. Extremely long lifetimes are found for the excitations of this mode, and the trajectories indicate that the experimental width is predominantly rotational. (3) The rocking and deformation modes are dominant receiving modes in the relaxation of the methyl stretch. (4) A shorter lifetime is calculated for the v=6 vs the v=5 or v=7 overtones of the methyl C–H stretch. Experimental results are qualitatively consistent with this prediction. The origin of this shorter lifetime is a band of resonances between the stretch excitation and combinations of rocking, deformation, and pseudorotation modes. (5) CH3 internal rotation figures importantly in the relaxation of some levels (v=5, 8 of CH3) where it "closes the energy gap'' for achieving resonant energy transfer. (6) For v=8 of the methyl CH, some direct energy transfer to both C–C(Triple Bond)C stretching modes is seen. The switching on of the stretches as receiving modes is a consequence of sufficiently strong interactions between the excited H and the C–C(Triple Bond)C chain, which take place at these high vibrational energies. (7) Evidence is found for long distance "through-space'' energy transfer due to long-range dipole–dipole forces. This transfer occurs from the acetylenic to the methyl CH stretches. This result is illustrated for the v=2 excitation of the acetylenichydrogen, and constitutes a direct demonstration of intramolecular long-distance, through-space v–v energy transfer. These results demonstrate the potential importance of large amplitude modes such as rocking and deformation as initial receiving modes for vibrational energy from excited CH overtones. On the time scale probed here (∼1 ps), despite the availability of many degrees of freedom, the transfer process is dominated by specific energy transfer channels and by the specific behavior of individual modes, rather than by statistical considerations, which will certainly prevail on longer time scales.

Notes: Molecular-dynamics (MD) simulations are used to study the vibrational properties of ICl adsorbed on an MgO(001) surface, and the photodissociation dynamics of the molecule after excitation to a 1Π electronic state. The electronic ground-state simulations show that ICl lies nearly parallel to the surface and occupies a single orientational site at surface temperatures below 150 K. Above 350 K the molecule hops between two orientational sites on the surface, and at 500 K full rotational diffusion of the adsorbate in the surface plane occurs. The multiplicity of sites and the onset of rotational diffusion at high T were found to greatly affect the dissociation dynamics and its temperature dependence. The photodissociation simulations show that only a fraction of the Cl atoms and some of the I atoms (which have a much higher binding energy) leave the surface following photolysis (at these energies). The fraction of Cl atoms leaving the surface subsequent to photodissociation at 50 K is ∼0.5, and it decreases as T is raised to 150 K. The trajectories show that Cl atoms leave the surface preferentially for initial ICl orientations in which the Cl end "points down.'' This orientation ensures that the escaping atom rapidly collides with the surface atoms. Momentum transfer due to surface local roughness is crucial for the Cl to acquire "escape velocity'' normal to the surface. The angular intensity distribution of the Cl atoms is sensitive to surface corrugation, and the energy distribution of the photofragments strongly reflects the Cl/surface collision stage of the process. It is concluded that photodissociation experiments can provide information both on surface local structure and on photofragment/surface interaction and energy transfer.

Notes: The photodissociation of an isolated IBr molecule adsorbed on an MgO(001) surface is studied theoretically. The calculations correspond to an excitation into the repulsive (Y0+) /quasibound (B 3Π0+) electronic state manifold, which may lead to the production of excited state or ground state bromine atoms. Using a quantum scattering method, we calculate the photoabsorption line shape for this process and the [Br]/[Br*] branching ratio as a function of photoexcitation wavelength. In the quantum calculations the IBr stretching was treated exactly, the in-plane librational mode was treated in the sudden approximation, and all other adsorbate and crystal modes were frozen. In addition, we studied the photodissociation process classically in order to explore the validity of freezing most of the modes. In the quantum calculations it was found that the width and intensities of the structured part of the absorption profile were greatly increased compared with the gas-phase photodissociation process. This was attributed to the stabilization of both electronic states by the molecule/surface interactions. The classical results showed that at least semiquantitatively, the crystal modes are unlikely to affect the process on the timescale pertinent to the calculated line shape.

Notes: Exact quantum mechanical and quasiclassical trajectory results for rotationally inelastic N2 -corrugated surface collisions are compared over the energy range 0.01–0.04 eV. It is found that the degeneracy averaged, diffraction summed, rotationally inelastic transition probabilities display quantum oscillations.

Notes: The self-consistent field (SCF) approximation for coupled anharmonic vibrations is applied to the calculation of vibrational energy levels (predissociation resonances) of collinear models of I2 (v)He and I2 (v)Ne, with vibrational quantum number v between 5 and 30. The predissociation lifetimes of these same states are obtained from a distorted wave Born approximation calculation with self-consistent field states taken as the initial states of the complex, and with correlation between the modes taken as the interactions leading to decay. Although the binding energies of the van der Waals complex are very small (order of several cm−1 ), the SCF eigenvalues are in remarkable agreement with the exact numerical values. The lifetimes obtained from the SCF-distorted wave Born approximation (DWBA) are compared with calculations in which the initial state is treated more simply, assuming separability of the modes involved. Results then show that the DWBA with SCF initial states is considerably more accurate than with the more primitive initial state choice. We conclude from these results that the self-consistent field method offers a very accurate description of large-amplitude vibrational motions in van der Waals clusters, with good quantitative results for both the energy levels and predissociation dynamics of these species.

Notes: The full anharmonic stretching and bending potential of CO2 is determined to high accuracy from the measured vibration/rotation spectrum. The calculation consists of two stages: first, a direct explicit inversion of the data within a semiclassical self-consistent-field (SCF) treatment of vibrational dynamics and second, a refinement of this result by a first-order perturbative approach that includes corrections to the SCF approximation, but assumes that the SCF-inverted potential shows only small deviations from the true surface. The final result is tested by comparison of the energy levels calculated exactly from the determined potential against the original experimental input. Based on this criterion of accuracy in reproducing experimental frequencies, the present inverted potential appears more accurate than previous potentials obtained from empirical fitting of the data. These results indicate that the perturbatively corrected SCF inversion method is a very powerful tool for obtaining potential energy surfaces directly from experimental data, with errors not exceeding several wave numbers.