Selectivity in coherent transient Raman measurements of vibrational dephasing in liquids

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Abstract
The microscopic information content of coherent transient Raman measurements is analyzed. It is shown that for short pulses and optically thin samples the Kaiser–Laubereau pulse sequence is the Raman analog of the optical free induction decay, and that the experimental observable contains the same dynamical information as the isotropic, spontaneous Raman line shape. Under these conditions, the experiment therefore cannot be used to selectively measure the homogeneous dephasing time of a system with an inhomogeneously broadened line. The results of our analysis are at variance with the earlier results of Kaiser and Laubereau and the more recent predictions of Oxtoby. However, these experiments, under certain circumstances, may be used to obtain a nonselective line narrowing, as found by Zinth, Polland, Laubereau, and Kaiser. We also consider the situation in which the sample is optically dense, in which case laser depletion must be taken into account. The distinction between saturation of the vibrational transition and depletion of the pump pulse is discussed, and selectivity is shown to arise from the former phenomenon, rather than from the latter. This result is at variance with the predictions of George and Harris. An alternative pulse sequence, the Raman echo, is suggested as a tool to achieve selectivity. A unified theory of the Raman echo is developed, which is valid for a bath with arbitrary time scale and which interpolates between the limits of homogeneous and inhomogeneous line broadening.