The probe frequency dependence of nonresonant femtosecond pump-probe nuclear responsese: Undercutting vibrational inhomogeneities

摘要

Expressions are derived for the intensity of nuclear responses appearing in the frequency dispersed optical heterodyne detected (OHD) birefringence and dichroism of nonresonant materials. The dispersed birefringence of chloroform is reported and the detuning dependence of the two intramolecular modes (260 cm~(-1) and 370 cm~(-1)) are compared with theory. These probe frequency dependent birefringence responses are pumped by a 40 fs pulse and probed with either a 40 fs transform limited Gaussian pulse (FWHH approx 400 cm~(-1)) or a one-sided exponential pulse (FWHH approx 36 cm~(-1), Lorntzian). Excellent agreement with theory is found. Due to the CARS and CSRS resonances inherent to these responses, the relative magnitude of different nuclear responses which contribute to the total response of a system can be selectively enhanced in the dispersed pump-probe response when the probe spectrum is narrower than the difference between the relevant mode frequencies. It is shown how this two-dimensional (time and frequency ) P~((3)) technique can be used as a measure of the extent of inhomogeneous broadening contributing to the impulsively excited low frequency intermolecular density of states by the appropriate choice of detection frequencies and pulse shape.