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.