Temporal coherence effects in multiphoton excitation of collisionless molecules

In this paper we examine the conditions for the erosion of coherence effects in the multiphoton excitation of a collision-free sparse molecular level structure driven by a laser field, which is characterized by a stabilized amplitude and a fluctuating phase. The stochastic equations of motion for the molecule excited by a phase-diffusing laser field were solved within the framework of second-order cumulant expansion. The kinetic master equation was found to be valid for the description of the dynamics of the multiphoton excitation process provided that the correlation time of the laser phase is considerably shorter than the relevant molecular time scales and that the rate of the slow diffusion of the laser phase considerably exceeds the Rabi frequencies. We conclude that the experimental observation of the celebrated fluence dependence of the efficiency of high-order multiphoton excitation in isolated molecules may sometimes originate from the fluctuations in the laser field rather than from intramolecular vibrational energy redistribution.