DOES SOFT-X-RAY LUMINESCENCE REFLECT THE DENSITY-OF-STATES OF THE VALENCE-BAND, EVEN IF CORE-EXCITON AND VALENCE-EXCITON EFFECTS EXIST

We theoretically study the core- and valence-excitation effects on spe ctra of soft X-ray radiations in an insulator. A three-band system com posed of a dispersionless core band, a conduction band and a valence b and, with wide energy gaps among them, is taken as a model for the ins ulator. By using this model, a resonant second-order optical process i s calculated. If there is a core-exciton effect alone, the radiation s pectra are not affected so much. On the other hand, if the core-and va lence-exciton effects coexist, and also if the incident energy resonat es to the core-exciton, very efficient optical conversion from the cor e-exciton to the valence-exciton occurs, and this results in a new pro minent peak. However, when the incident energy resonates to the conduc tion band itself, the spectral shape reflects the van Hove singulariti es of a density of states of the valence band. Thus, even if the core- and the valence-exciton effects exist, we can obtain sufficient infor mation on the density of states from the soft X-ray radiation spectra by choosing the incident energy appropriately. (C) 1998 Elsevier Scien ce B.V. All rights reserved.