Nonlinear Optical Response of Metal Surfaces with Adsorbed Molecules

摘要

A quantum mechanical theory describing the photodesorption dynamics of molecules bonded to metal surfaces and irradiated by pulsed laser light is presented. The physical system is separated into a primary region comprising the adsorbate and the bonding substrate atoms and a secondary region consisting of the remaining substrate. The two regions, which are strongly coupled, interact in a self-consistent manner via an electric dipole-electric dipole coupling. A density matrix approach, in which the density operator obeys the Liouville-von Neumann equation, is used to study the dynamics of photodesorption. An indirect desorption mechanism is considered and a local electromagnetic field, which induces electron-hole excitations in the metal, is introduced. The substrate response to femtosecond laser irradiation, including nonlinear effects of the laser electric-field strength, is investigated through solutions of the optical Bloch equations for the radiation field-substrate interaction and generalizes previous work for weak perturbing fields. An effective Hamiltonian which exhibits a nonlinear dependence on the field strength and includes the effects of dissipation is obtained. The theory developed is then parameterized using numerical values for the CO/Cu(001) adsorbate-substrate system and illustrated with model calculations. These show that the effective potential in the primary region is very sensitive to variations in the electric-field strength.