Nonlinear optical response and yield in the femtosecond photodesorption of CO from the Cu(001) surface: A density matrix treatment

The dynamics of molecular photodesorption from a metal surface is described by a density matrix theory of the nonlinear optical response resulting from the interaction of a femtosecond pulsed laser with a metal surface. The extended system is divided into a primary region comprising the adsorbate species and the bonding substrate atoms and a secondary region consisting of the remaining substrate, that interact strongly and self-consistently with each other through an electric dipole-electric dipole coupling. The formalism uses the Liouville-von Neumann equation, with an effective Hamiltonian which includes the effects of energy dissipation into the metal. The nonlinear response of the substrate is studied by solving the optical Bloch equations with relaxation terms to account for the effects of energy dissipation, coupled to kinetics equations describing the excitation of the electron Fermi sea by the light pulse. A primary effective hamiltonian with a nonlinear dependence on the electric field strength of the laser is obtained as a result. The theory is applied to the CO/Cu(001) adsorbate-substrate complex and the nonlinear photodesorption yield of CO versus pulse fluence is evaluated through model calculations. The local electric field at the adsorbate, and the yields for several fluence values are obtained as functions of the desorption time. (C) 2002 American Institute of Physics.