Photodesorption by resonant laser-vibrational coupling: Effects of coherent two-quantum transitions, vibrational anharmonicity, and surface heterogeneity on yields

Photodesorption may be initiated by resonant, laser-induced excitation of an internal vibrational mode of an adsorbed molecule. The desorption kinetics are predicted from a quantum-statistical theory based on a master equation describing phonon- and photon-assisted molecular transitions as well as elastic tunneling processes. Explicit contributions to desorption by coherent two-phonon and by one-photon–plus–one-phonon absorption and emission are shown to be important, while coherent two-photon processes are typically negligible. The calculated line shape of the photodesorption yield can differ from the corresponding infrared absorption spectrum due to nonlinear intensity effects, vibrational anharmonicity, and surface heterogeneity. A qualitative comparison with published experimental results on the adsorption systems ${\mathrm{CH}}_3$F on NaCl, pyridine on KCl, and Co and NaCl is presented.