Vibrational substructure in the OH stretching transition of water and HOD

Ultrafast nonlinear vibrational spectroscopy with mid-IR pumping and incoherent anti-Stokes Raman probing is used to study ν=1 excitations of OH stretching (VOH) of water and of HOD in D ₂O solvent (HOD/D ₂O). The parent VOH 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 ν _OH band, the excited-state spectrum can be fit by two overlapping subbands, a broader red-shifted band ν _OH ^R and a narrower blue-shifted band ν _OH ^B. We show these subbands are dynamically distinguishable. They decay with different lifetimes and evidence characteristically different decay pathways. Excitations of the ν _OH ^R subband generate bending vibrations that ν _OH ^B does not. The shorter lifetime (0.5 ps) of the ν _OH ^R subband compared to the ν _OH ^B 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 ν _OH ^R 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 ν _OH ^R subband has a smaller amplitude in HOD/D ₂O than in water, possibly because HOD has a single localized OH-stretching vibration whereas water has two delocalized stretching vibrations.