Topics in Condensed Matter Physics: the Role of Exchange in the Lithium K Edge and the Fluorescence Spectrum of Heavily Doped Cadmium-Sulphide

The theory of X-ray thresholds was extended to include exchange effects which are found to reduce threshold exponents and increase the orthogonality index. Results of detailed numerical calculations of threshold exponents for Li are presented which differ considerably from previous values and which are in better agreement with edge data. A model was developed to explain the fluorescence spectrum of heavily doped n-type cadmium sulphide on the high density side of the metal-insulator transition. This model, based on the assumption that the impurity electrons form a metallic band, predicts a well defined upper threshold for the spectrum whose location is in good agreement with the data. The model is also consistent with the other qualitative features of the spectrum.