Vibrational couplings and ultrafast relaxation of the O–H bending mode in liquid H 2O

Abstract

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.