Quantum studies of methane-metal inelastic diffraction and trapping: The variation with molecular orientation and phonon coupling

Abstract

Quantum mechanical methods are used to compute the trapping and gas-phonon energy exchange for CH 4 incident on Ir(1 1 1). The gas-surface interaction and all properties of the phonons are computed from first principles. The gas-surface interaction is strongly dependent on molecular orientation. Orientations amenable to C H bond dissociation exhibit a large amplitude “anti-corrugation”, and diffraction probabilities for different CH 4 orientations can vary by orders of magnitude. The phonon coupling is shown to be generally soft and long ranged, and also dependent on molecular orientation. Orientationally-averaged values for the corrugation and phonon coupling are used to describe scattering in the rotational ground state, leading to good agreement with recent experiments. However, experiments that can selectively orient the incident molecule should observe diffraction, trapping and gas-phonon energy transfer that vary with alignment. We derive a Baule-like model for energy transfer using our computed phonon coupling and use it to explain our scattering results for different molecular orientations. Applying these ideas to Pt(1 1 1) and Ni(1 1 1) we find that the gas-Pt interaction is relatively hard while the gas-Ni interaction is very soft and long ranged. Energy transfer should thus be weaker than expected for the methane-Ni(1 1 1) system, consistent with molecular dynamics studies.