Coherence effects in the anisotropy of optical experiments

It is often assumed that the anisotropy in time-resolved fluorescence and pump-probe experiments is always smaller than or at most equal to 25. From resonance Raman experiments it is well known, however, that the anisotropy can range anywhere from 1 to −12 (depolarization ratio between 0 and ∞) depending on the symmetry of the final state in the Raman transition. Since the resonance Raman intensity depends on the short-time vibrational dynamics in the excited state potential, it is to be expected that similar anisotropies will be observed in femtosecond pump-probe experiments. In this article the anisotropies in ultrashort pulse pump-probe experiments will be calculated and the original results, well known from the theory of resonance Raman, are recovered for zero delay times. In time-resolved pump-probe experiments there is a new effect, however, that is not present in resonance Raman scattering: for increasing delay times the anisotropy is expected to decay due to the electronic dephasing of the degenerate electronic states involved. Therefore measurement of the femtosecond anisotropy decay enables one to obtain electronic dephasing times in condensed phases.