Coherent effects in femtosecond infrared spectroscopy

The accessible time resolution in femtosecond infrared experiments is shorter than the typical phase relaxation time of a vibronic transition. Therefore, coherent interaction of the light pulses with the sample may disturb the observed absorbance signals. Coherence results in an artifact known as perturbed free induction decay, which may be misinterpreted as an intrinsic incoherent temporal evolution of the sample. In the present paper, a model is presented describing this effect for the general situation, where a complex molecule containing many overlapping vibrational modes is investigated. The model leads to an efficient linear least square fit algorithm allowing the analysis of huge data sets. The model and the fit algorithm are applied to transient absorbance changes observed in a large dye molecule. It is demonstrated that it is possible to separate an ultrafast energy relaxation process from the perturbed free induction decay signal. In addition, the analysis of the perturbed free induction decay effect itself allows one to obtain information on the instantaneous absorbance change of the sample.