A THEORETICAL-STUDY OF THE PHOTODESORPTION OF NO PT(111)/

Time-dependent quantum wavepackets have been used in a model calculati on to investigate the substrate-mediated photodesorption of a molecule from a metal surface. A ''hot'' electron, generated in the substrate by an absorbed photon, temporarily resonates in an unoccupied molecula r orbital. This results in a new set of forces, and if the electron sp ends sufficient time in the resonance, then on returning to the electr onic ground state the molecule will have acquired sufficient energy to desorb. We have not employed the Born-Oppenheimer approximation, but rather studied the dynamics on potential energy surfaces (PESs) explic itly including both the electronic and nuclear coordinates. PES parame ters were chosen to model NO desorption from Pt where it has been sugg ested that the excited state is a ''negative molecular ion'', which is attracted to the surface. The desorption probability has been calcula ted as a function of hot electron energy for different potential topol ogies. We show that observable desorption is possible for short resona nce lifetimes (< 5 fs). In order to make contact with experiment, resu lts have been convoluted with an effective electron distribution that describes the cascade process.