The principal research aim is the detailed experimental and theoretical investigation of the structure and properties of transient species, such as weakly bound Van der Waals molecules and free radicals. High resolution spectroscopic work is performed in the near collision-free environment of a molecular beam using the techniques of microwave Fourier transform spectroscopy and infrared diode laser absorption spectroscopy.
1. Van der Waals molecules and hydrogen bonded complexes
The weak attractive forces between molecules are often sufficiently strong to permit the formation of weakly bound dimers (and higher complexes) known as Van der Waals molecules. Although usually present in only small proportions, they can be formed in large concentrations by supersonic expansion of the appropriate gas mixture through a small orifice. The spectroscopic investigation of such species provides valuable information on the nature of the intermolecular forces involved.
Using molecular beam spectroscopic techniques we are probing the structure and dynamics of a variety of complexes. Binary complexes between simple molecules, including the more strongly bound hydrogen-bonded complexes, are being investigated. We are also beginning to investigate larger complexes to test simple theories of the packing of molecules (so important in the condensed phases).
2. Atmospheric clusters
The atmosphere contains many very small clusters such as aerosols and the species in the polar stratospheric clouds. It is well known that such particles have a profound effect on the chemistry of the atmosphere, but many of their properties are poorly understood. For example, because of their small size and high surface area to volume ratio, their properties can differ significantly from those of the bulk phases. We have been studying clusters between water and oxides of nitrogen and sulfur. Early results show that the binary complex of water and sulfur trioxide is not sulfuric acid.
3. Interactions in biological systems
Intermolecular forces greatly influence the structure and properties of biological systems. Although many of these "properties" have been intensively modelled, it is not always clear that the forces responsible have been fully understood. In particular we are currently studying the clusters of chiral molecules in order to investigate the stereospecific nature of the interactions between such molecules.
4. Open-shell complexes
While the interaction between two free radicals usually leads to the formation of a chemical bond, the interaction between an open-shell and a closed-shell molecule is less well understood. However such interactions are obviously involved in the collisions of radicals with other molecules. In order to investigate such effects we have been obtaining extensive spectra of Van der Waals complexes containing open-shell molecules such as Ar-NO2 and NO-HF. These spectra give the structure of the complexes and hyperfine interactions provide information on any electron reorganisation which has occurred during complex formation.
5. Free radicals and ions
Most of the spectroscopy of highly reactive radicals and ions has been limited to very small molecules containing no more than 3 or 4 atoms. However, with the advantages of the very low temperatures of supersonic nozzles we hope to study significantly larger species. For this we have been developing a range of laser photolysis and electric discharge nozzle sources which produce "cold" radicals and ions. Since the resulting molecular beam environment is similar to that of interstellar space we plan to produce and study a range of astrophysically significant molecules.
6. Theory of intermolecular interactions and the determination of potential energy surfaces
The forces between molecules, despite their importance in determining many of the properties of matter, are still very poorly understood. We have developed a number of theoretical models to predict the interaction between molecules. We also use results from the above experiments on molecular complexes to help determine accurate anisotropic potential energy surfaces.