Matthew F. Vernon

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...

Vibrational predissociation spectra of (HF)n, n = 2–6

Using molecular beam techniques and a tunable infraredlaser, the vibrational predissociation spectra for (HF)n, n = 2 to 6, in the 3000 to 4000 cm−1 range are presented. The vibrational bands have been assigned to intramolecular HF stretching modes and combinations of intra‐ and intermolecular modes. The structures of (HF)n, n = 3 to 6, were found to be cyclic, i.e., each HF molecule is both a proton donor and acceptor.

Reactive scattering of Na(3 2P3/2)+HCl

The reaction of electronically excited Na(3P) atoms with HCl has been studied in a crossed molecular beams experiment. At collision energies slightly above the endoergicity of the reaction, Na(3P) shows a dramatic enhancement of reactivity over ground state Na(3S). Detailed measurements of the laboratory angular and velocity distributions of the reactively scattered NaCl product at 5.4 kcal/mol collision energy have allowed determination of the product center‐of‐mass translational and angular distributions. These experimental results are compared to the DIPR model of electron transfer reactions. The broad translational energy distribution is in qualitative agreement with the DIPR model, but the angular distribution exhibits reduced intensity for scattering perpendicular to the relative velocity vector which cannot be reproduced by the DIPR model. The preferred transition state configuration, Na–Cl–H, is consistent with what would be predicted by a diffuse 3P orbital where the Na atom appears ion‐like. Thi...

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.

A decoupled rigid bender model for the anharmonic force field of octahedral molecules

Abstract The molecular vibrational Hamiltonian for an octahedral molecule is derived to fourth order, subject to the constraint that the bond lengths are held constant. By assuming that the force field for the bending motions is a points-on-a-sphere potential, the location of the first difference bands for the pure bending motions is calculated; mode couplings are neglected. With these results, systematic trends in the anharmonic shifts can be predicted for the complete family of metal-hexafluorides. The effects of mode couplings and the importance of these calculations for understanding the initial process in the multiphoton dissociation of SF 6 are briefly discussed.

Infrared Vibrational Predissociation Spectroscopy of Small Molecular Clusters

The structure and interaction of small molecular clusters have been the subject of extensive studies. Molecular beam electric resonance spectroscopy, Fourier-transform microwave spectroscopy and pressure induced infrared absorption spectra have provided information concerning the ground state structure and the intermolecular potential energy surface for a large number of binary systems. Recently, vibrational predissociation experiments using molecular beam techniques and tunable infrared lasers have measured the infrared absorption spectra and dynamical properties of many van der Waals and hydrogen bonded clusters.