Electron-lattice interaction in ionic and partially ionic solids

We examine the interaction of electrons with the lattice in solids where the resulting influence on electronic states is so large ($>10\mathrm{}{k}_B\Delta T$) that the process cannot properly be explained in terms of phonon scattering. A general description is given of temperature effects on photoemission so that we could properly use this tool to study electron (and vibrational) states. We propose a microscopic description in terms of a dynamic modulation of the orbital hybridization. While transition matrix elements may be effected, proper magnitudes for experimental peak broadening and its temperature dependence are calculated on the basis of density of states modulation alone. This phenomenon can be used to identify the atomic origin of certain photoemission (and optical absorption) peaks, as demonstrated for $d$ states in the noble-metal halides (and KBr). General criteria are established for a solid to exhibit such strong (yet not Frank-Condon) electron-lattice coupling. Generally speaking, the valence states of solids which have an ionicity midway between pure covalent and pure ionic ($0.86>\mathrm{}{f}_i>0.73\mathrm{}$), and certain conduction states of more ionic materials ($1>\mathrm{}{f}_i>0.86\mathrm{}$) should exhibit the necessary dynamic hybridization. We also consider effects of this interaction on the vibrational states.