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

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. © 1982 American Institute of Physics

Vibrational predissociation of benzene dimers and trimers by the crossed laser-molecular beam technique

A molecular beam composed predominantly of benzene monomer, dimer, and trimer is excited with tunable, pulsed, infrared radiation in the C-H stretch frequency range (3000-3100 cm-1). Two types of experiments are performed for the observation of the vibrational predissociation of the excited cluster, measuring directly the wavelength dependence of the predissociation yield and the translational energy distribution of the predissociation products. The wavelength dependence of the vibrational predissociation is found to be similar to the infrared spectra of room-temperature liquid benzene. The translational energy distributions of the predissociation products are used to deduce dynamical properties for the following predissociation mechanism: (C6H6)n + hν → (C6H6)n-1 + C6H6** (τ), where ** indicates vibrational excitation. The lifetime, τ, of the vibrationally excited clusters is determined to be in the range of 10-12 < τ < 10-6 s. The most probable predissociation is characterized by all of the excess energy appearing in the rotational and vibrational motions of the products. The two product molecules do not seem to share this excess energy equally, the monomer product retaining in excess of 2/3 of the available energy. These observations are qualitatively consistent with current theories of vibrational predissociation of weakly bound molecular clusters. © 1981 American Chemical Society

Vibrational predissociation spectra and dynamics of small molecular clusters of H2O and HF

Experimental results are presented for the vibrational predissociation spectra in the frequency range 3000-4000 cm-1 for the species (HF)n and (H2O)n, n = 2-6, using molecular-beam techniques and a tunable infrared laser. The observed spectra show a dramatic change between the dimer and larger clusters which is thought to be a result of the cyclic structure of the trimer and larger clusters. The spectra are compared with calculated harmonic force constants of available intermolecular potentials to understand how these small, gas-phase clusters relate to the liquid and solid phases of HF and H2O. Additionally, the angular distributions of the predissociation products show that little energy appears as translational motion of the fragment molecules. This conclusion is consistent with recent theoretical models of the predissociation process. An upper limit of ca. 2 μs is observed for the lifetime of the vibrationally excited clusters

Zeitschrift für pädagogische Historiographie : ZpH

We describe recent experiments in which mass spectrometry and laser spectroscopy are combined to characterize Li(+)-H(2), Na(+)-H(2), B(+)-H(2) and Al(+)-H(2) complexes in the gas-phase. The infrared spectra, which feature full resolution of rotational sub-structures, are recorded by monitoring M(+) photofragments as the infrared wavelength is scanned. The spectra deliver detailed information on the way, in which a hydrogen molecule is attached to a metal cation including the intermolecular separation, the force constant for the intermolecular bond and the H-H stretching frequency. The complexes all possess T-shaped equilibrium geometries and display a clear correlation between the length and force constant of the intermolecular bond and the dissociation energy. In contrast, the data do not support any straightforward correlation between the frequency shift for the H-H stretch mode and the dissociation energy