Influence of the pure vibrational dephasing processes on the dip structure in pumped sum-frequency generation spectroscopy

Recent theoretical analysis of infrared-visible pumped sum-frequency spectroscopy on adsorbed molecules exhibits a dip structure on the frequency dependence of the sum-frequency signal intensity. To get more physical insight into this dip, which has not yet been observed experimentally? we present a unified description capable of handling, on the same footing, steady-state and pulsed experiments. Contrary to previous treatments, this description is valid for any duration of the dephasing times. Therefore, it enables a detailed discussion about the origin of the dip, as well as a determination of the conditions of its observability. These conditions are, in fact, very sensitive to the purl vibrational dephasing processes. In the nonresonant conditions, usually adopted in these experiments for the visible probe beam, the purr: electronic dephasing does not affect the dip structure and the underlying dynamics. However, if the visible probe beam becomes resonant with the electronic transition, the purr electronic dephasing introduces a strong asymmetry of the dip. [S1050-2947(99)00505-3].