The results of a photoelectron study using 40.81-eV photons of the outermost bands of the group-IIB-metal fluorides ZnF"CdF, , and HgF, are reported. The relaxation-corrected experimental values for the energy separation and absolute binding energies of the outermost levels and the corresponding values given by the fully ionic crystal model are found to be in good agreement for CdF, and approximate agreement for HgF, . Assignment of relaxation energies for ZnF"of similar magnitude to those for HgF, , also results in approximate agreement between experimental data for this compound and the corresponding ionic-model predictions, The valence-band widths of a number of MF and MF, compounds, including rocksalt, fluorite, rutile, arid tetragonal crystal types, are shown to be a function of d -' (d is the bond length) and are characterized by a valence-band index that is independent of crystal type. I~INTRODUCTIONThis paper reports the results of a photoelectron study of the outermost electronic bands of the . group-IIB-metal fluorides ZnF"CdF"and HgF, .These materials are of interest as part of a wider study of the class of nominally strong ionic materials. This study has so far included the alkalimetal halides, "' the alkaline-earth fluorides, ' some transition-metal fluorides' all of which have been successfully interpreted in terms of a fully ionic crystal model, and a number of soft-metal fluorides which in sharp contrast to the above materials were found"' to have electronic structure characteristic of covalent electronic states. The present work extends these investigations to the group-IIB-metal fluo rides.'The electronic structures of CdF, and HgF, are also of interest as examples of materials which crystallize in the fluorite structure (ZnF, crystallizes in a rutile structure). Materials which crystallize in the fluorite structure have in the past been of considerable -interest'; however, very little experimental data are available on the electronic structure of these materials and until very recently no energy-band calculation has been performed. ' II. EXPERIMENTAL The experimental details in regard to the spectrometer parameters, energy calibration, sample -preparation, and method of determining binding energies have been published in Ref. 7 and in references contained therein. Details of the photon source have been published in Ref. 9. Briefly,ultraviolet photons from a helium discharge lamp, operated under conditions which optimize the production of 40,81-eV radiation, strike a freshly evaporated target, and the photoelectrons are energy analyzed by a 90"-sector spherical electrostatic analyzer. The effects of sample charging were accounted for by recording several spectra for each sample within the first 30 s after evaporation; binding energies at time zero were determined from the spectra by an extrapolation procedure, ' as discussed previously. ' III. THEORY