Chapter 33 - Three-dimensional spectroscopy of vibrational energy relaxation in liquids

Abstract

Vibrational energy relaxation (VR) in liquids is a fundamental process in chemical physics that remains poorly understood. When a parent vibration is excited, it typically decays on the picosecond time scale. In polyatomic molecules with many vibrational states, excitation of a higher frequency parent vibration leads to a complicated chain of events termed vibrational cooling (VC). In VC, parent decay could excite one or more daughter vibrations. The leftover energy is transferred to collective excitations of the liquid. Ultimately, all the daughter and higher energy collective excitations will decay in secondary processes resulting in a bulk increase in the thermodynamic temperature by an amount ∆T. In the past several years, a number of multidimensional vibrational spectroscopy techniques have been used to study liquids. Most of these techniques involve vibrational coherences, so they are of limited use in probing VR. This chapter uses a three-dimensional vibrational spectroscopy technique with mid-IR pumping and incoherent anti-Stokes Raman probing to study vibrational energy relaxation (VR) in hydrogen bonded liquids. New results on spectral diffusion and vibrational relaxation of OH stretching vOH of water are presented. Vibrational energy transfer down a molecular chain is probed with angstrom spatial resolution in a series of alcohols where VQH is pumped and energy transfer through methylene -CH2- to terminal methyl -CH3 groups is probed.