The low-frequency spectra of simple liquids have been studied with a stimulated Raman gain technique using tunable picosecond pulses in the mid-infrared region. The obtained results on CS2, CCl4, C2Cl4 and CDCl3 are rescaled to reduced Raman spectra, which are a representation of the low-frequency density of states of these liquids, weighted by the corresponding intermolecular polarizability. This thermal 'bath' is believed to accept or provide small portions of energy during vibrational relaxation processes. The technique demonstrated here has a large potential for time-resolved studies of vibrational relaxation and related phenomena. (C) 2003 Elsevier Science B.V. All rights reserved.