Quantum–stochastic approach to laser-stimulated desorption dynamics and population distributions of chemisorbed species on solid surfaces

A quantum–stochastic treatment is presented for multiphoton processes of chemisorbed species on solid surfaces. The average excitation is evaluated for different stochastic processes. In the Markovian fast modulation limit, the average excitation takes a Lorentzian form, while taking Gaussian in the slow modulation limit. The addition of the memory effects is calculated by Mori’s projection operator technique and is found to be a function of the correlation time and the frequency modulation of the stochastic processes. For low excitations, the average excitation is found to be linearly proportional to the laser intensity, and is saturated by the red shift of the anharmonicity for high excitations. The overall broadening of the infrared line shape is caused by the fluctuation of the adspecies-field interaction energy, the excitation-induced surface migration, the phonon-dispersion-induced dephasing, and the energy relaxation due to the adspecies–phonon coupling. The population distributions for the slow T1 relaxation rate are characterized by a Poisson-type distribution (governed by the average excitation), and are characterized by a Boltzmann thermal distribution for the fast T1 relaxation rate (governed by both the average excitation and the vibrational degrees of freedom of the adspecies). A comparison of laser-stimulated desorption and the effective thermal desorption is quantitatively shown by introducing an effective temperature. Finally, a chemisorbed–physisorbed state–state transition channel with lower desorption energy (and hence enhancing the desorption probability) is proposed.