Vibrational couplings and ultrafast relaxation of the O-H bending mode in liquid H2O

The femtosecond dynamics of O-H bending excitations are studied in pump-probe experiments at frequencies from 1000 to 3500 cm(-1). Upon resonant O-H bending excitation, one finds a v = 1 -> 2 absorption extending over similar to 200 cm(-1) and a v = 1 lifetime of 170 fs. O-H bending relaxation transfers excess energy to intermolecular modes, giving rise to a red-shifted librational absorption. Upon O-H stretching excitation, the O-H bending oscillator shows an instantaneous response due to a Fermi resonance between the v = 1 stretching and v = 2 bending states. The coupled states decay with a 200 fs lifetime via the O-H bending and librational modes. (c) 2006 Elsevier B.V. All rights reserved.