Extreme-Ultraviolet Spectra of Ionic Crystals

The absorption spectra of several ionic crystals were obtained by the use of synchrotron radiation with photon energies in the range 50-250 eV. This range includes thresholds for excitation of both $p$ and $d$ core states. Arguments are given that peaks in the observed spectra are generally due to maxima in the final density of states, rather than exciton phenomena. The chlorine $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ spectra of NaCl, KCl, RbCl, and even AgCl are very much alike, which can be understood in terms of similar conduction-band structure for these materials. It is suggested that double excitations are not as important as collective effects. The $3d\to p$ spectra of the ${\mathrm{Br}}^{-}$ Kr-${\mathrm{Rb}}^{+}$ sequence, as well as the $4d\to p$ spectra of the ${\mathrm{I}}^{-}$-Xe-${\mathrm{Cs}}^{+}$ sequence, can be understood by taking into account the spin-orbit splitting of the initial states and the final-state band structure. Very prominent $d-\mathrm{t}\mathrm{o}-f$ resonances were found for compounds containing iodine and cesium. In CsCl and CsBr, structure near 160 eV due to excitation of the cesium ${\mathrm{N}}_{\mathrm{III}}$ level shows an unusual antiresonance behavior.