Ultrafast nonlinear vibrational spectroscopy with mid-IR pumping and incoherent anti-Stokes Raman probing is used to study v = 1 excitations of OH stretching (νOH) of water and of HOD in D2O solvent (HOD/D2O). The parent νOH decay and the appearance of daughter stretching and bending excitations are simultaneously monitored, which allows for characterization of the stretch decay pathways. At all times and with all pump frequencies within the νOH band, the excited-state spectrum can be fit by two overlapping subbands, a broader red-shifted band and a narrower blue-shifted band . We show these subbands are dynamically distinguishable. They decay with different lifetimes and evidence characteristically different decay pathways. Excitations of the subband generate bending vibrations that does not. The shorter lifetime (∼0.5 ps) of the subband compared to the subband (0.8−0.9 ps) results primarily from enhanced stretch-to-bend anharmonic coupling. The subbands represent persistent structures in the excited state, in that interconversion between subbands (2−10 ps) is slower than excited-state decay. A tentative structural interpretation is proposed. The subband, on the basis of simulations, its red shift ,and its shorter lifetime, is proposed to result from strongly hydrogen-bonded “ice-like” water. The subband has a smaller amplitude in HOD/D2O than in water, possibly because HOD has a single localized OH-stretching vibration whereas water has two delocalized stretching vibrations.

In a suspension of reverse micelles, in which the surfactant sodium dioctyl sulfosuccinate (AOT) separates a water nanodroplet from a bulk nonpolar CCl4 phase, ultrafast vibrational spectroscopy was used to study vibrational energy transfer from the nanodroplet through the AOT interfacial monolayer to the surrounding CCl4. Most of the vibrational energy from the nanodroplet was transferred to the polar AOT head group within 1.8 picoseconds and then out to the CCl4 within 10 picoseconds. Vibrational energy pumped directly into the AOT tail resulted in a slower 20- to 40-picosecond transfer of energy to the CCl4.

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Anti-Stokes Raman scattering is used to monitor vibrational energy redistribution in the ambient temperature liquids nitromethane (NM-h3) and perdeuterated nitromethane (NM-d3) after ultrafast IR excitation of either the symmetric or asymmetric CH- or CD-stretch transitions. The instantaneous populations of most of the fifteen NM vibrations are determined with good accuracy, and a global fitting procedure with a master equation is used to fit all the data. The pump pulses excite not only CH- or CD-stretches but also certain combinations of bending and nitro stretching fundamentals. The coupled vibrations that comprise the initial state are revealed via the instantaneous rise of the anti-Stokes transients associated with each vibrational fundamental. In contrast to many other polyatomic liquids studied previously, there is little energy exchange among the CH-stretch (or CD-stretch) excitations, which is attributed to the nearly free rotation of the methyl group in NM. The vibrational cooling process, which is the multistep return to a thermalized state, occurs in three stages in both NM-h3 and NM-d3. In the first stage, the parent CH- or CD-stretch decays in a few picoseconds, exciting all lower-energy vibrations. In the second stage, the midrange vibrations decay in 10-15 ps, exciting the lower-energy vibrations. In the third stage, these lower-energy vibrations decay into the bath in tens of picoseconds. The initial excitations are thermalized in approximately 150 ps in NM-h3 and there is little dependence on which CH-stretch is excited. VC is somewhat faster in NM-d3 with more dependence on the initial CD-stretch, taking approximately 100 ps with symmetric CD-stretch excitation and approximately 120 ps with asymmetric CD-stretch excitation. Comparison is made with earlier nonequilibrium molecular dynamics simulations of VC [Kabadi, V. N.; Rice, B. M. Molecular dynamics simulations of normal mode vibrational energy transfer in liquid nitromethane. J. Phys. Chem. A 2004, 108, 532-540]. The simulations do a good job of reproducing the observed VC process and in addition they predicted the slow interconversion among CH-stretch excitations and the slower relaxation of the asymmetric CH-stretch now observed here.