Vibrational excitation of adsorbed molecules by low-energy photon-emitted electrons: A dynamical model

Abstract: A simple, inelastic electron-scattering dynamical model is presented to account for vibrational excitation in molecular adsorbates. The basic two ingredients of the theoretical model are: (i) the conservation of the total angular momentum, and (ii) the requirement of a critical time to allow for the intra-molecular energy re-arrangement of the transient negative-ion complex. The model is applied to the vibrational excitation dynamics of molecules chemisorbed at sub-monolayer conditions on ordered metal surfaces. This was exemplified for Acrylonitrile adsorbed on Cu(100), whose vibrational excitation was studied via energy loss spectra of low-energy two-photon photoemission (2PPE) electrons, and for ammonia (NH3 and ND3) adsorbed on Cu(100), being probed in a STM experiment. Fits of the model to the data allowed for deducing the energy threshold of the vibrational excitation of the Ce:glyph name="tbnd" />N bonds of the ACN adsorbate molecules, and the threshold for the symmetric ν 1-stretch mode excitation of adsorbed NH3/ND3. Also, information about the temporal dynamics underlying the inelastic electron scattering was gained. [Copyright &y& Elsevier]