ELECTRONICALLY DRIVEN ADSORBATE EXCITATION MECHANISM IN FEMTOSECOND-PULSE LASER-DESORPTION

Femtosecond-pulse laser desorption is a process in which desorption is driven by a subpicosecond temperature pulse of order 5000 K in the su bstrate-adsorbate electron system, whose energy is transferred into th e adsorbate center-of-mass degrees of freedom by a direct coupling mec hanism. We present a systematic theoretical treatment of this coupling process in the language of an electronic friction, which generates La ngevin noise in the adsorbate center-of-mass degrees of freedom, while the electronic degrees of freedom are at a high temperature. Starting from an influence-functional path-integral description, a simple form ula for the electronic friction is defined which is valid at all elect ronic temperatures. At low temperatures the formalism makes contact wi th the electronic friction appearing in the theory of adsorbate vibrat ional damping, whereas at high temperatures comparable with the adsorb ate electronic excitation energies the friction becomes strongly tempe rature dependent due to dominance by virtual excitations between diffe rent adsorbate potential energy surfaces. The former regime is related to the electronic friction model for the desorption process, and the latter to the desorption induced by multiple electronic transistions m odel for the process; the present formulation comprises both regimes. Desorption is calculated both by a simple quasianalytic Kramers rate a pproach, and by numerical solution to the Langevin equation. The magni tude of the desorbed fraction and the time scale for desorption are co mpared to experimental results.