Infrared Vibrational Predissociation Spectroscopy of Water Clusters by the Crossed Laser Molecular Beam Technique PDF Download

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Languages : en
Pages : 75

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Water clusters formed in a molecular beam are predissociated by tunable, pulsed, infrared radiation in the frequency range 2900~3750 cm−1. The recoiling fragments are detected off axis from the molecular beam using a rotatable mass spectrometer. Arguments are presented which show that the measured frequency dependent signal at a fixed detector angle is proportional to the absorption spectrum of the clusters. It is found that the spectra of clusters containing three or more water molecules are remarkably similar to the liquid phase spectrum. Dynamical information on the predissociation process is obtained from the velocity distribution of the fragments. An upper limit to the excited vibrational state lifetime of ~1 microsecond 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. Both the time scale and translational energy distribution are consistent with the qualitative predictions of current theoretical models for cluster predissociation. From adiabatic dissociation trajectories and Monte Carlo simulations it is seen that the strong coupling present in the water polymers probably invalidates the simpler "diatomic" picture formulations of cluster predissociation. Instead, the energy can be extensively shared among the intermolecular motions in the polymer before dissociation. 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 are 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. The observed differences between the dimer and larger polymers (trimer-hexamer) indicate a dramatic change in the hydrogen bonding, which is best explained as arising from the non-additive effects present when a water molecule is both donating and accepting a hydrogen bond. This difference between dimer and trimer also rationalizes the previous disagreement between potential functions based on condensed phase properties (where the water molecule is interacting with multiple neighbors) and those fit to imperfect gas or dimer properties which sample only the isolated pair potential. The data support an interpretation of the hydrogen bonded O-H stretching fundamental region as arising from a homogeneous broadening (not necessarily a result of the predissociation) whose width is characteristic of the hydrogen bond itself and not the sum of distinct bonding geometries. This is different from some previous theories of the water infrared absorption spectrum which assign each band to water molecules bound to different numbers of neighboring molecules.