Density matrix theory and calculations of nonlinear yields of CO photodesorbed from Cu(001) by light pulses

The photodissociation of CO from Cu metal surfaces due to absorption of vis ible and ultraviolet (UV) light pulses is described within a density matrix approach, including the nonlinear optical response of the substrate to pul ses of large fluence. We introduce a self-consistent coupling of adsorbate and substrate regions, and treat the substrate as a stochastic medium to ac count for dissipative effects following its electronic excitation. Our mode l is based on potential energy surfaces, couplings, and transition dipoles parametrized from electronic structure calculations for CO/Cu. The dynamics of photodesorption is obtained propagating wave packets with a nonperturba tive treatment which includes the time dependence of the light pulse. Resul ts have been obtained for the time evolution of state populations, and for yields of CO versus pulses fluence, with a range of values of the pulse wid th and light wavelength and of the dissipation time constant. Our numerical results for the desorption yields and desorption times are consistent with results of femtosecond photodesorption experiments at both low and high fl uence values. (C) 1999 American Institute of Physics. [S0021-9606(99)70221- 7].