The infrared predissociation spectra of water clusters

Abstract

Abstract Coker, Miller, and watts carried out experimental measurements of the infrared predissociation spectra of small water clusters in molecular beams formed with free jet expansions of water vapor in helium. The spectra include several O-H stretch absorptions and an H-O-H bend overtone. Theoretical analyses based on empirical intramolecular/intermolecular potential energy surfaces were made using normal mode theory, local mode theory, and a novel extension of the simple random-walk procedure without importance sampling. The normal mode analysis was found inadequate and the local mode analysis not quite satisfactory, but the QMC-based quantum simulation procedure predicted the observed vibrational bands for the dimer and trimer very accurately. The simple QMC method was used to generate a large number of walkers with a distribution corresponding to the ground state wavefunction 7/J0. This many-body function was then projected onto the local vibrational coordinates for each molecule to give single-variable functions, each then fit by a Morse function. A variational calculation including cross terms for low-lying excited states was then made with this reference state. The results gave unambiguous assignments for the observed O-H stretching vibrations in the dimer, good agreement for the bending overtones, and satisfactory agreement for the O-H vibrations in the trimer. Repetition using a different pair interaction potential established that the experiment could provide a sensitive method for testing pair interactions.