Vibrational energy redistribution in polyatomic liquids: 3D infrared–Raman spectroscopy

Abstract

Vibrational energy relaxation (VER) in polyatomic liquids and solutions can be studied in unprecedented detail using ultrafast incoherent anti-Stokes Raman spectroscopy to monitor the decay of a parent vibration generated with a tunable mid-IR pump pulse, and the appearance of daughter excitations at lower frequencies. A 2D vibrational spectroscopy technique results from fixing the mid-IR pump pulse and obtaining a time series of anti-Stokes Raman spectra. A 3D technique results from obtaining 2D spectra as a function of pump frequency. This 3D technique is presently the most powerful method available for studying VER in condensed phases. It provides information on VER mechanisms and pathways, liquid-state structural relaxation resulting in spectral diffusion, excitation transfer among sites with different local structures, strong correlations between vibrations as a result of Fermi resonance, and extremely weak anharmonic interactions resulting in VER via multistep vibrational cascades. The technique is illustrated by recent results obtained with ambient temperature associated liquids water (and deuterated water) and methanol, and by new measurements of strong correlations between CH stretching and bending and OH stretching and bending vibrations in methanol.