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 ∼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.