QUANTUM DYNAMICS OF BOND BREAKING IN A DISSIPATIVE ENVIRONMENT - INDIRECT AND DIRECT PHOTODESORPTION OF NEUTRALS FROM METALS

The dynamics of uv/visible laser-induced nonthermal desorption of neut ral molecules from metal surfaces are studied by Liouville-von Neumann equations for quantum open systems. A one-dimensional, two-state Gadz uk-Antoniewicz model is adopted, representative for NO/Pt(111). Electr onic quenching due to coupling of the adsorbate negative ion resonance to the metal electrons is treated within the Lindblad dynamical semig roup approach. Both indirect (hot-electron mediated) and hypothetical direct (dipole) excitation processes are considered. For the indirect pathways, DIET (single-excitation) and DIMET (multiple-excitation) lim its are studied using one- and double-dissipative channel models, resp ectively. To reproduce experimental desorption yields and desorbate tr anslational energies, we estimate the quenching lifetime for NO/Pt(111 ) to be less than 5 fs. We also extend previous quantum treatments of photodesorption processes to the case of coordinate-dependent quenchin g rates. Further, the characteristic scaling laws of desorption yields versus laser fluence are derived for each of the individual excitatio n pathways. Finally, the possibility to control photoreactivity at sur faces by different, vibration-promoted schemes (surface heating, ir+uv two-photon strategies, use of pulsed uv lasers) is examined. (C) 1996 American Institute of Physics.