The optical properties of orthorhombic sulphur in the vacuum ultra violet.

This thesis describes an experimental study of the reflectivity spectrum of orthorhombic sulphur crystals in the vacuum ultra violet spectral range, extending to photon energies of 14 eV.. The construction and design of the spectrometer and of the associated apparatus is described in some detail; experimental techniques as well as several problems encountered in vacuum ultra violet measurements are discussed. An essentially new type of reflectometer has been developed for the present work, which gives accurate and reliable results at near normal incidence. Reflectivity results were obtained for a range of crystals with natural (111) faces that had been subjected to widely different surface treatments. It is concluded that the reflectivity spectra are largely independent of surface treatment and represent the intrinsic optical properties of orthorhombic sulphur. Reflectivity spectra have been determined for Germanium specimens to check the reliability of the instrument and experimental technique. The Kramers-Kronig relation has been used to analyse the reflectivity data and values of the optical constants n and k have been derived for photon energies up to 14 eV.. A discussion is given of the general nature of the Kramers-Kronig analysis, and a suitable procedure for processing the sulphur data chosen. The internal consistency of the calculations has been confirmed by the use of sum rules. Where possible, the computed data have been compared to experimental results. The absorption coefficient in particular shows good correlation with experimental single crystal and thin film work. An interesting feature of the absorption is the "window" at 8 eV. which may enable thin films of sulphur to be used as band pass filters in this region. The relation that exists between certain functions of the optical constants and band structure is indicated. The generated values of the complex part of the dielectric constant, E2, are analysed and the results tentatively interpreted in terms of band structure. The difficulties of such an interpretation are brought out, but the information obtained is in substantial agreement with a qualitative scheme proposed by Gibbons. The results will be of some importance when more detailed theoretical calculations become available.