Spectra, Dynamics, and Intermolecular Forces in Small Molecules and Clusters

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Knowledge of the intermolecular forces or interaction energies within and between molecules is essential for understanding and making predictions about properties of systems ranging from simple dimers to dilute gases to bulk liquids or to solids. Developing and applying methods for determining and representing such interactions, and utilizing such information to study simple molecules, both isolated and in clusters or condensed phase environments, is the central theme of this proposal. 1) Diatomic Molecule Modelling, Data Analyses, and Computational Methods. This project is centred around the development and application of the 'direct potential fit' method of data analysis, in which a comparison of experimental spectra with the results of an exact quantum simulation based on a flexible analytic model potential energy function is used to optimize parameters defining both the model potential function and the atomic mass-dependent Born-Oppenheimer breakdown correction functions required to make the data for different isotopologues consistent with one another. This approach was first introduced in our (1974) treatment of three-dimensional atom-diatom Van der Waals molecules, and its applications to diatomics have been developed over the past 20 years in a number of laboratories around the world. It provides more fundamental physical information, and yields much better extrapolation and interpolation predictions than do conventional 'parameter-fit' analyses. However, the limitations of some of the potential forms used and the complexity of the method have inhibited its general adoption. Our work has included the development of a better way of solving the radial Schroedinger equation that allows levels lying extremely close to dissociation to be located and treated just as easily as levels lying deep in a potential well. It has also included the development and testing of improved potential function models that incorporate the correct theoretically-known long-range behaviour. Planned work includes an extension of this model to allow it to better deal with non-standard 'potential shelf', double-minimum or barrier behaviour. 2) Simulations of Dynamics and Vibrational Shifts in Molecular Clusters and Matrices. The most discriminating experimental probe of the physical properties and behaviours of molecular ('Van der Waals') clusters are the spectra of chromophores seeded into or onto the cluster. However, to exploit fully the experimental work requires a quantitative understanding of how the spectroscopic properties of the chromophore molecule are affected by its surroundings. Understanding and predicting the vibrational frequency shifts of molecules trapped within or on the surface of solids is a similar problem, since both are governed by the same types of microscopic intermolecular interaction and frequency-shift mechanisms. Quantitative understanding of such shifts would facilitate probing local order in and on the surface of materials, determining the phase behaviour (liquid-like vs. solid-like) of finite molecular clusters, and examining solvation behaviours in cluster-type systems. Our previous work included the development of a successful model for quantitative prediction of the splitting and shifting of IR spectra of a certain type of chromophore in a cluster environment; it showed that such spectra could provide distinct signatures for cluster size, structure and phase behaviour. We are now applying path-integral quantum Monte Carlo methods to simulate the properties and spectroscopic signatures of infrared chromophores in molecular clusters, and have found that they provide sharp tests of subtleties of the interactions and the nature of the dynamical behaviour and possible presence of superfluidity.