Infrared vibrational predissociation spectroscopy of water clusters by the crossed laser-molecular beam technique

Abstract

Water clusters formed in a molecular beam are predissociated by tunable, pulsed, infrared radiation in the frequency range 2900–3750 cm−1. Absorption spectra of the clusters are obtained by detecting the recoiling fragments off-axis from the molecular beam as a function of laser frequency using a rotatable mass spectrometer. By carefully adjusting the expansion conditions of the molecular beam and monitoring the largest cluster observable, excessive contamination by clusters larger than the specific one of interest is avoided. It is found that the spectra of clusters containing three or more water molecules absorb over the same frequency range as the liquid. Dynamical information on the predissociation process is obtained from the measured angular and velocity distributions of the fragments. An upper limit to the excited vibrational state lifetime of ∼1 μs is observed for the results reported here. The most probable dissociation process concentrates the available excess energy into the internal motions of the fragment molecules. From adiabatic dissociation trajectories and Monte Carlo simulations it is seen that the strong coupling present in the water polymers causes extensive energy sharing among the intermolecular motions in the polymer before dissociation, consistent with the experimentally measured energy distributions. Comparison between current intermolecular potentials describing liquid water and the observed frequencies is made in the normal mode approximation. The inability of any potential to predict the gross spectral features (the number of bands and their observed frequency shift from the gas phase monomer) suggests that substantial improvement in the potential energy functions is possible, but that more accurate methods of solving the vibrational wave equation are necessary before a proper explanation of the spectral fine structure is possible.