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 transitio n. 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 misinte rpreted as an intrinsic incoherent temporal evolution of the sample. I n the present paper, a model is presented describing this effect for t he general situation, where a complex molecule containing many overlap ping vibrational modes is investigated. The model leads to an efficien t linear least square fit algorithm allowing the analysis of huge data sets. The model and the fit algorithm are applied to transient absorb ance changes observed in a large dye molecule. It is demonstrated that it is possible to separate an ultrafast energy relaxation process fro m the perturbed free induction decay signal. In addition, the analysis of the perturbed free induction decay effect itself allows one to obt ain information on the instantaneous absorbance change of the sample.