We introduce a discrete-continuum hybrid treatment of solid vibrations in order to describe the collisional excitation of adsorbate and defect modes by atom impacts. The inhomogeneous surface is represented by: (a) one or more atom clusters corresponding to the defect sites and their immediate neighbors, which are harmonically coupled to (b) an elastic continuous bulk. The model thus aims at reproducing the long-wavelength spectrum of the lattice as well as the high-frequency localized modes contributed by adsorbates and surface defects. The hybrid model is tested against lattice results in one-dimensional simulations that allow for analytic solution of the surface motion (which would be unfeasible for three-dimensional imperfect lattices); hybrid and lattice results are thus compared in detail under identical conditions. The model is also evaluated under the worst possible conditions for the continuum approximation, since collinear collisions correspond to three-dimensional situations in which the transferred momentum and, therefore, short-wavelength excitations are maximal. Comprehensive tests are presented for He atoms scattering from CO chemisorbed on Pt and on Ni substrates, and from N 2 adsorbed on W. The scattering dynamics is treated by time-correlation functions of the transition operator previously developed for polyatomic targets [J. Chem. Phys. 84, 3162 (1987); 85, 2300 (1987); 86, 750 (1987)]. All the energetically open states are thus incorporated, as well as the thermal average over initial states, without need of basis-set expansions. Distributions of transferred energy are presented as would be observed in a one-dimensional scattering "experiment" using impact energies between 0.1 and 1 eV and with the solid at nonzero temperature. The collisional spectra obtained from the hybrid models are found to be in excellent agreement with those of the corresponding inhomogeneous lattices throughout wide ranges of impact velocity, surface initial temperature (including 0 K) and transferred energy. The results indicate that discrete-continuum treatments provide a powerful tool for analyzing the transfer of energy at the gas-adsorbate-solid interface.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry