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 (nu(OH)) lifetime for water and HDO is similar to 1 ps. The OD stretch (nu(OD)) lifetime for D2O is similar to 2 ps. Stretch decay generates substantial excitation of the bending modes. The lifetimes of bending vibrations (delta) in H2O, HDO, and D2O can be estimated to be in the 0.6 ps less than or equal to T-1 less than or equal to 1.2 ps range. nu(OH) decay in water produces delta(H2O) With a quantum yield 1.0 less than or equal to phi 2.0. In HDO/D2O solutions, v(OH)(HDO) decay generates nu(OD)(D2O). delta(HDO), and delta(D2O). The quantum yield for generating nu(OD)(D2O) is phi approximate to 0.1. The quantum yield for generating both delta(HDO) and delta(D2O) is phi greater than or equal to 0.6. Thus, each nu(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 similar to 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.