Vibrational energy relaxation and spectral diffusion in water and deuterated water

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 (ν_OH) lifetime for water and HDO is ∼1 ps. The OD stretch (ν_OD) 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 ≤ T₁ ≤ 1.2 ps range. ν_OH decay in water produces δ_H2o with a quantum yield 1.0 ≤ φ ≤ 2.0. In HDO/D₂O solutions, ν_OH(HDO) decay generates ν_OD(D₂O), δ_HDO, and δ5_D2o. The quantum yield for generating ν_OD(D₂O) is φ ≈ 0.1. The quantum yield for generating both δ_HDO and δ_D2O is φ ≥ 0.6. Thus, each ν_OH(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.