A study of the optical, surface and photoemissive properties of orthorhombic sulphur.

This thesis describes a number of experimental studies relating to the surface properties of orthorhombic sulphur in the near and vacuum ultraviolet, in the energy range 3-14 e.v. The optical absorption coefficient using a thin film technique at 80°K has been measured up to photon energies of 6.2 e.v. This modifies previous results taken over a more limited energy range and correlates well with a band model of the absorption processes in this region proposed by Spear and Adams. An examination of surface transits has shown anisotropy in charge separation in the excited region and is identified with an anomalous photovoltaic effect first described Ruppel and Grant. Further observations on the photovoltaic response are made and possible causes are discussed. Photoconductivity and photoemission measurements have been made over the entire energy range and are correlated to produce a tentative band model. This confirms the essential correctness of the previous work by Cook and Spear and adds new data on intraband structure and the nature of the higher lying conduction bands. The model predicts the onset of high mobility electron generation at 4.9 e.v. The behaviour of an insulating solid under photoemission has proved to be very different from that of a metal. These characteristics have been analysed from a theoretical standpoint with results that are consistent with the observed phenomena. Theoretical calculations include those made on the nature of the potential and field distribution in a surface cell which explains some of the experimental results in this thesis and certain reported anomalies by other authors, and on the charge distribution during carrier transits when subject to the diffusive action of stochastic trapping and release.