A Novel Technique for the Measurement of Polarization-Specific Ultrafast Raman Responses

Abstract

A simple time domain method for the observation of polarization-specific Raman responses in electronically nonresonant materials is demonstrated. When a cutoff filter is placed in the probe beam path before the detector in the conventional pump−probe configuration, the in-phase dichroic optical heterodyne-detected (OHD) response is enhanced as compared to the usual putative corresponding dichroic response observed when the probe is not dispersed. The ultrafast excited OHD responses of CS2 obtained by this method are reported for parallel, perpendicular, and magic angle relative orientations of pump and probe pulse polarizations. The observed dispersed dichroic signal can be derived from the real part alone of the third-order nuclear response function. The decay of the CS2 isotropic response is found to be dominated by a ∼500 fs decay process for times longer than ∼0.7 ps. This relaxation time scale matches the nondiffusive exponential decay seen in the birefringent and dichroic anisotropic responses of CS2. Calculated instantaneous normal mode (INM) isotropic and anisotropic nuclear response functions are found to exhibit exponential decays in this same 500−600 fs time scale, suggesting that this decay component may be predominantly determined by the distribution of Raman-weighted density of states.