Ultrafast vibrational relaxation of O-H bending and librational excitations in liquid H2O

Population relaxation of the O-H bending mode in pure liquid H2O is studied via nonlinear vibrational absorption in mid-infrared pump-probe experiments with a 100 fs time resolution. After resonant excitation of the v = 0 → 1 bending transition, a v = 1 lifetime of 170 ± 30 fs is measured. Such fast relaxation is due to anharmonic couplings to solvent modes. It is followed by slower spectral diffusion and energy redistribution processes within 1-2 ps. Femtosecond excitation also induces ultrafast nonlinear changes of the broad librational absorption occurring at frequencies below the v = 0 → 1 bending transition.