Observation of low-frequency Raman and Kerr effect contributions in picosecond infrared pump probe experiments

It is shown experimentally and theoretically that in nonlinear infrared spectroscopic studies applying laser pulses of approximate to 2 ps duration or shorter quite different resonant and nonresonant processes can give rise to significant transient intensity changes in the probe beam. Particularly stimulated Raman gain and loss due to low-lying vibrational resonances and, near-zero frequency, intermolecular interactions and librational modes are observed in liquid CCl4, CS2, C2Cl4, and a dilute solution of CHBr3 in the latter. It is discussed that in two color pump probe experiments these effects can reach the same order of magnitude as changes due to transient excess population, particularly when dilute solutions are studied. Additionally a novel device for comprehensive single-shot polarization analysis is described, and its usefulness for the investigation of the real part of chi((3)) is demonstrated. (C) 2000 Elsevier Science B.V. All rights reserved.