Surface Second Harmonic GenerationThe in situ study of biologically and technologically relevant buried or difficult-to-access interfaces (e.g. solid/liquid, liquid/liquid) requires special techniques to ensure that the molecules at the interface can be studied in isolation from those in the bulk phases.
The non-linear spectroscopic technique of second harmonic generation (SHG) provides the required selectivity, with sub-monolayer sensitivity, and yields information on the concentration and orientation of adsorbed species. Using this technique my group has looked, for example, at the way the orientation of adsorbed species such as dyes, peptides and complexes, depend on the polarity of the solvent at how adsorbed species can be influenced by applied fields and, by using a flow cell, at the temporal evolution of liquid-liquid interfaces. The results of my research have demonstrated that it is essential to record the full polarisation behaviour of the SHG fields, which led me to develop an ellipsometric technique as an efficient and effective way of obtaining this information. In order to interpret the SHG data we carry out high level ab initio calculations on frequency-dependent molecular hyperpolarisibilities. We combine the ab initio calculations with orientation and conformations from molecular simulations at the air/liquid interface to predict the SHG results. I have also made significant contributions to the understanding of non-linear chiral effects and our study of small peptides at liquid interfaces has shown the importance of the large circular differential scattering in the optical response of such systems. These studies have opened the way to the investigation of the secondary and tertiary structure of proteins within a membrane by spectroscopic techniques.