Ultraviolet Absorption of Insulators. IV. Rare-Gas Solids

The analysis of the fcc alkali halides given in III is extended to the rare-gas solids, especially Xe. The interband spectrum is similar to the alkali halides. The valence bands of Xe are narrow compared to the spin-orbit splitting, which produces extra structure in the one-electron spectrum. The orbital valence band width at $L$ is 0.5 eV, in agreement with band calculations. The lower lying conduction bands are nearly free electron in character. Because the crystal is monatomic with only neutral lattice defects, the coherent exciton spectrum is well resolved. Again, an intimate connection is established between the interband edges and both hydrogenic and extra excitons (those not associated with the edge at $\Gamma$). Exciton-exciton configuration interactions mediated coherently by the photon field and incoherently (dissipatively) by the phonon field are studied qualitatively. According to the general scattering theories of Fano and Van Hove, certain relations are expected to hold between diagonal and off-diagonal elements of the exciton-photon and exciton-phonon self-energies. It is demonstrated qualitatively that these relations hold, and that they account not only for asymmetries of individual resonance lines but also for the entire pattern of resonances and antiresonances in Xe. In particular, previously unexplained anomalies in the Rydberg spectra of the "halogen doublets" are shown to be direct consequences of exciton-exciton configuration interaction mediated by the coherent photon field.