Three-Dimensional Spectroscopy of Vibrational Energy Relaxation in Liquid Methanol

Abstract

Vibrational energy relaxation (VER) of neat methanol at ambient temperature is studied with a three-dimensional vibrational spectroscopy technique. The first two dimensions are represented by a time series of incoherent anti-Stokes Raman spectra at a given mid-IR pump frequency. The third dimension involves changing the mid-IR pump frequency within the manifold of CH and OH stretching transitions ν(CH) and ν(OH). Each 2D representation shows the VER dynamics occurring as a result of pumping a specific vibrational transition. The decay of the pumped transition into first-generation daughters, and the subsequent decay into second-, and even third-generation daughters can be monitored. The third dimension shows how VER depends on the nature of the pumped transition. Additional information about the buildup of excitation in the bath of lower-energy collective excitations (phonons) is obtained by monitoring the heating of CCl4 spectators in methanol−CCl4 mixtures. Three distinct stages are seen in VER of ν(CH). The pumped ν(CH) creates two quanta of bending vibrations δ(CH) or δ(OH) (0−1 ps), the δ(CH) vibrations preferentially create CO stretch ν(CO), and the δ(OH) create methyl rock ρ(CH3) (2−5 ps), and then ρ(CH3) and ν(CO) decay into phonons (5−15 ps). Three stages are also seen in ν(OH) VER except that the first stage involves exciting every other vibration to some extent. About 10% of ν(OH) decay occurs in four stages, where the first stage is ν(OH) → ν(CH). The IR−Raman 3D technique is compared to other multidimensional vibrational spectroscopies. This 3D method is presently the most powerful technique for studying VER in condensed phases. It is sensitive to overtone and combination transitions buried under more intense fundamental absorption spectra. In addition it is sensitive to very weak cascaded anharmonic couplings responsible for the excitation of successive generations of daughter vibrations.