A novel technique for the measurement of polarization-specific ultrafast Raman responses

A simple time domain method for the observation of polarization-specific Ra man responses in electronically nonresonant materials is demonstrated. When a cutoff filter is placed in the probe beam path before the detector in th e conventional pump-probe configuration, the in-phase dichroic optical hete rodyne-detected (OHD) response is enhanced as compared to the usual putativ e corresponding dichroic response observed when the probe is not dispersed. The ultrafast excited OHD responses of CS2 obtained by this method are rep orted 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 respons e function. The decay of the CS2 isotropic response is found to be dominate d by a similar to 500 fs decay process for times longer than similar to0.7 ps. This relaxation time scale matches the nondiffusive exponential decay s een in the birefringent and dichroic anisotropic responses of CS2. Calculat ed instantaneous normal mode (INM) isotropic and anisotropic nuclear respon se functions are found to exhibit exponential decays in this same 500-600 f s time scale, suggesting that this decay component may be predominantly det ermined by the distribution of Raman-weighted density of states.