Epioptical modelling

• Main Author: Roseburgh, David Stewart
• Published: University of Edinburgh 2006
• Subjects: 541.33
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.661381

We have modelled epioptic systems using semi-empirical methods. We apply one such theoretical technique, Berreman’s 4x4 matrix formalism, to simulate and compare the results of various epioptic spectroscopies when applied to model systems. We place particular emphasis on 45° reflectivity spectroscopy (45DR), and demonstrate its potential for the study of systems with surface regions of mixed isotropic and anisotropic character. We also demonstrate the superior surface senstiviity of 45DR when compared to spectroscopic ellipsometry (SE) and show that 45DR is inherently more stable with regards to inaccuracies in bulk data. However, we also reveal that practical considerations lessen the potential impact of this technique. We continue the application of this matrix formalism to simulate the absorption of molecular species onto model surfaces. We show that reflection anisotropy spectroscopy (RAS) is sensitive to both molecular orientation and substrate properties, and that a simple polarisable dipole model, omitting local field contributions, enables us to reproduce the experimental spectra of Goletti <i>et al</i> and Weightmann <i>et al</i>, who studied porphyrin and DNA base molecule absorption respectively. We also assess the effect of optical activity on RAS spectra. Finally we apply the derivative model, in which surface optical spectra are related to perturbations of bulk electronic transitions, to noble metal (110) RAS spectra. We demonstrate that this model describes, consistently and accurately, the spectra of these intrinsically anisotropic systems. Furthermore, we show that observed transition broadening in ion-bombarded systems can be explained in terms of quantum mechanical uncertainty derived from roughness induced localisation of near-surface electrons.