Quantum molecular dynamics in intense laser fields: Theory and applications to diatomic molecules

Abstract

The quantum dynamics of vibration–rotation excitation in diatomic molecules in intense laser fields is investigated. The Floquet method is used in solving the equations-of-motion for a Hamiltonian explicitly time-dependent. This method requires computation of the time-displacement propagator only over the first optical cycle of the laser field, and is accomplished both numerically and with Magnus approximations. A number of features of single and multiphoton absorption in the LiH, CO, IBr, and HF molecules are studied as functions of the laser intensity and frequency. Average photon absorption spectra are studied with respect to power broadening, dynamic Stark shifts and line shapes. In some cases effective two-state perturbative results accurately agree with the numerical results. In addition, rotational distributions in IBr following two-photon absorption are found to have both thermal components for nonresonant states and structured nonthermal components for states nearly in resonance with the field. Finally, the intensity and temperature dependence of a two-photon laser induced isotopic enrichment in CO is studied. The 14CO to 13CO enrichment ratio is found to decline with increasing laser intensity and initial rotational temperature.