In the broad water stretching band (2900−3700 cm-1), frequency-dependent vibrational energy relaxation (VER), and spectral diffusion both occur on the time scale of a few picoseconds. Ultrafast IR−Raman spectroscopy of water is used to study both processes. VER is also studied in solutions of HDO in D2O (HDO/D2O). The OH stretch (νOH) lifetime for water and HDO is ∼1 ps. The OD stretch (νOD) lifetime for D2O is ∼2 ps. Stretch decay generates substantial excitation of the bending modes. The lifetimes of bending vibrations (δ) in H2O, HDO, and D2O can be estimated to be in the 0.6 ps ≤ T 1 ≤ 1.2 ps range. νOH decay in water produces δH 2 O with a quantum yield 1.0 ≤ φ ≤ 2.0. In HDO/D2O solutions, νOH(HDO) decay generates νOD(D2O), δHDO, and δD 2 O. The quantum yield for generating νOD(D2O) is φ ≈ 0.1. The quantum yield for generating both δHDO and δD 2 O is φ ≥ 0.6. Thus, each νOH(HDO) decay generates at minimum 1.2 quanta of bending excitation. After narrow-band pumping, the distribution of excitations within the stretch band of water evolves in time. Pumping on the blue edge instantaneously (within ∼1 ps) generates excitations throughout the band. Pumping on the red edge does not instantaneously generate excitations at the blue edge. Excitations migrate uphill to the blue edge on the 0−2 ps time scale. The fast downhill spectral diffusion is attributed to excitation hopping among water molecules in different structural environments. The slower uphill spectral diffusion is attributed to evolution of the local liquid structure. Shortly after excitations are generated, an overall redshift is observed that is attributed to a dynamic vibrational Stokes shift. This dynamic shift slows down the rate of excitation hopping. Then energy redistribution throughout the band becomes slow enough that the longer VER lifetimes of stretch excitations on the blue edge can lead to a gradual blue shift of population over the next few picoseconds.

Vibrational energy relaxation and vibrational cooling of polyatomic liquids were studied with the ultrafast infrared-Raman (IR-Raman) technique. In the IR-Raman technique, a type of two-dimensional vibrational spectroscopy, a vibrational transition is pumped with a mid-infrared pulse and the instantaneous populations of all Raman-active transitions are simultaneously probed via incoherent anti-Stokes Raman scattering of a time-delayed visible pulse. The theoretical framework for these measurements, including force-force correlation function methods and perturbative techniques, is reviewed. Experimental aspects of the IR-Raman technique are discussed, including laser instrumentation, experimental set-up, the nature of the pumping and probing processes, detection sensitivity and optical background, and the interpretation of results including spectroscopic artifacts. Then examples are provided from recent research by our group, focusing on timely problems such as the pseudo-vibrational cascade, the dynamics of doorway vibrations, dynamics of overtones with Fermi resonance, multiple vibrational excitations via combination band pumping and spectral evolution in associated liquids.

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