Threshold dependence of the vibrational excitation of molecules on laser radiation intensity

摘要

The collisionless vibrational excitation of a polyatomic molecule in an IR laser radiation field has been theoretically studied. It has been shown that (i) the degree of vibrational excitation (namely, number 0000 of vibrational quanta of a molecular mode near-resonant with the IR laser field that are absorbed by the molecule) is low if laser pulse intensity P (energy flux density in the laser beam) is lower than a certain critical value P (cr); (ii) the degree of excitation abruptly increases after crossing the boundary where P = P (cr); (iii) this effect is attributed to two properties inherent in polyatomic molecules, namely, the anharmonicity of the vibrational mode interacting with the laser field and the energy exchange with other modes; and (iv) at P > P (cr), number 00000 is determined only by energy density I broken vertical bar = P tau(P), where tau(P) is the laser pulse duration, 00000 monotonically increases with increasing I broken vertical bar. The model is in good agreement with the experimental data.