The state of information about vibrational population relaxation in the liquid state is still rather scarce. By using incoherent anti-Stokes scattering after strong infrared excitation, it is now possible to obtain rather complete information about these relaxation processes. In this paper we will report a study on the chloroform molecule. As all vibrations of this molecule are Raman active, they can be investigated by this experimental approach. Population lifetimes between 23 and several hundred ps are found. This is, to our knowledge, the first measurement displaying simultaneously the time evolution of all fundamentals of a molecule after direct population of a vibrational state.

Efficient intermolecular energy transfer between the C–Br stretching modes ν2 and ν5 of bromoform molecules in the liquid phase has been observed directly in time-resolved experiments applying vibrational pump–probe spectroscopy with picosecond laser pulses. An analysis of results on CHBr3, CDBr3, and an isotopic mixture of both yields a typical time constant of 25±15 ps for this rather efficient, near-resonant intermolecular relaxation channel. Additional new details about the intramolecular pathways of vibrational energy relaxation have been determined for the pure substances.

The use of Stokes difference spectra for picosecond studies of vibrational relaxation dynamics in condensed matter is introduced and compared to the established techniques of measuring anti-Stokes scattering or IR transmission changes. A simple quantum mechanical description reveals the principal similarities and differences of the different probing processes for the case of moderately anharmonic oscillators, e.g., liquids with only relatively weak interactions of the sample molecules. The outcome is illustrated by novel experiments on neat liquid chloroform, whose relaxation is known quite well. It is shown that transient Stokes data allow additional information, particularly about the behavior of low-lying vibrational levels and the type of energy transfer during relaxation, to be obtained.

Coherent antiStokes Raman spectroscopy of shockcompressed liquid nitrogen/carbon monoxide mixturesRecently the anti-Stokes spectroscopy after strong vibrational excitation via resonant absorption was rediscovered, for the study of population relaxation dynamics. A closer theoretical inspection of this experimental approach reveals that properly chosen polarization conditions allow the study of molecular reorientational motion. Furthermore the sign of a/␥ ͑the invariants of the Raman polarizability tensor͒ can be determined. Experimental data for chlorinated alkanes will be presented.

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