Ultrafast vibrational relaxation of liquid H2O following librational combination band excitation

Dispersed pump-probe responses excited by ∼165 fs pulses resonant with the bend-libration combination band in neat H2O centered at 2130 cm−1 are reported. This is the first IR pump-probe study of the energy relaxation dynamics in this region of the liquid water spectrum. The observed transient responses are best described by a kinetic model where the initial combination band energy relaxes via a concerted process to a bend fundamental and low frequency modes. The bend subsequently decays to another intermediate vibrational level, presumably the librational band, before this energy is distributed to low frequency modes and appears as heat as well in the sample volume in accordance with the results of Lindner et al. [L. Lindner, P. Vohringer, M.S. Pshenichnikov, D. Cringus, D.A. Wiersma, M. Mostovoy, Chem. Phys. Lett. 421 (2006) 329]. The lifetime of the bend-libration combination band is determined to be 140 ± 15 fs and the lifetime of the libration levels, leading to the hot ground state from the delayed heating route, is 840 ± 100 fs. This libration decay rate is consistent with the results of previous IR pump-probe measurement analyses for the appearance of heat following excitation resonant with the bend or stretch fundamentals. An additional pulse width limited absorption component is seen in all responses which may be attributed to the underlying continuum of intermolecular states in this region of the H2O spectrum.