Non-adiabatic approximation for resonant multiphoton excitation of vibrational-rotational states in strong laser fields

A method is presented for efficient and accurate treatment of infrared multiphoton absorption (MPA) processes in strong fields. By means of adiabatic separation of the fast vibrational motion from the slow rotational motion, one first solves the quasi-vibrational energy states (QVESs) with molecular orientation fixed. This reduces the computationally often formidable Floquet matrix analysis to a manageable scale. One then introduces non-adiabatic angular couplings of the adiabatic QVESs for proper analysis of resonant MPA. The utility of the method is illustrated by a study of the non-linear MPA quantusn dynamics in CO.