Electronic structure of beryllium fluoride

Vacuum-ultraviolet reflectance, absorptance, and photoelectron spectroscopy of vitreous and crystalline BeF/sub 2/ are reported. The data are interpreted with reference to self-consistent unrestricted Hartree-Fock cluster calculations also presented in this paper. The first allowed exciton reflectance peak in crystalline BeF/sub 2/ is at 12.9 eV, similar to the 12.8-eV peak found in the glass. Optical transmission of bulk samples extends at least to 9.5 eV, and can be assumed to be impurity limited in available material. Calculations of several defect and impurity levels are presented. Since the calculations presented here indicate that BeF/sub 2/ should have an optically forbidden band edge similar to that in SiO/sub 2/, the ultimate transparency range of purified BeF/sub 2/ will depend on the forbidden exciton absorption, not yet observable above the impurity background. Grazing-incidence reflectance spectra near the Be/sup 2 +/ K edge are interpreted in terms of a core exciton state lying at or slightly below the conduction-band minimum as determined from x-ray photoelectron data and the optical band gap.