Classical Model of Laser-Stimulated Surface Processes: Energy Absorption Profiles via the Langevin Equation

摘要

A classical model system, consisting of a diatomic molecule chemisorbed on a solid surface and subjected to infrared laser radiation, is presented. A set of coupled equations of motion characterized by the many-body effects of the surface atoms is reduced to the Langevin equations of a two-body problem in transformed coordinates. The surface-induced damping factor and frequency red-shift of the pumped mode are introduced by using a Wigner-Weisskopf-type approximation. The asymmetric forms of the power absorption and the quantized cross section due to the nonlinear effects of the anharmonicity are shown. The energy absorption profiles (energy absorbed vs time), which are universal for any ranges of laser intensity (i.e., as the intensity changes the profile remains the same provided the time scale is changed appropriately), are plotted for different sets of the damping factor and the detuning. It is found that much longer time scales (microsecond) are required for low-power excitations than for the high-power cases. The advantages and difficulties of the normal-mode method and the numerical method are discussed, and a new set of coupled equations in the rotating frame are developed.