TIME EVOLUTION OF CO VIBRATIONAL POPULATIONS DURING PHOTODESORPTION BY LIGHT-PULSES

We describe the direct photodesorption of CO from the Ni(001) metal su rface during the absorption of a short pulse of UV Light by the adsorb ate and account for the dynamics of the desorbing species coupled to e lectronic excitations of the substrate, which lead to energy dissipati on. The interaction potentials and couplings for the ground and excite d states are obtained from electronic structure calculations and from experimental information. The time evolution of CO vibrational populat ions is studied for propagating wavepackets for the adsorbate-substrat e complex with a split-operator algorithm, followed by a perturbative treatment of dissipation and response to the light pulse. Direct photo desorption is found to occur predominantly in the excited electronic s tate and is compatible with dissipation. Results are presented for CO photodesorption by Gaussian pulses with several durations to show the effect of light pulse shapes on the time evolution of populations. Sho rter pulses in the femtosecond range are shown to give relatively larg er populations of excited vibrational states during the desorption of CO. (C) 1997 John Wiley & Sons, Inc.