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 substrate-adsorbate electron system, whose energy is transferred into the adsorbate center-of-mass degrees of freedom by a direct coupling mechanism. We present a systematic theoretical treatment of this coupling process in the language of an electronic friction, which generates Langevin 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 formula for the electronic friction is defined which is valid at all electronic temperatures. At low temperatures the formalism makes contact with the electronic friction appearing in the theory of adsorbate vibrational damping, whereas at high temperatures comparable with the adsorbate electronic excitation energies the friction becomes strongly temperature dependent due to dominance by virtual excitations between different 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 model for the process; the present formulation comprises both regimes. Desorption is calculated both by a simple quasianalytic Kramers rate approach, and by numerical solution to the Langevin equation. The magnitude of the desorbed fraction and the time scale for desorption are compared to experimental results.