FREE ATOMS AND THE SIMPLEST ATOMIC CRYSTALS

The simplest atomic crystals, i.e. rare-gas solids, are qualitatively different from other dielectrics by a high atomic ionization potential which inhibits the formation of covalent bindings and provides a simp le electronic structure: an excited weakly bound electron moves in the free space between inert atoms placed in the lattice sites. The tradi tional self-consistent scheme takes no account of the strongly differe nt motion conditions for the outer electron and the tight bound electr ons of the lattice atoms. It is just this difference which is used in the present review to construct the adiabatic approximation: the fast atomic electrons are adjusted to an instantaneous position of the slow outer electron. The electronic excitations of the rare-gas crystals a re adequately described by the adiabatic theory which is simultaneousl y developed for a large class of free atoms. The crystal properties ar e derived From the atomic constants (ionization potentials and polariz ability), the new features originating from the crystalline state brin g pointed out. The adiabatic theory differs from known model approache s by the inferential criterion of applicability which guarantees a cer tain calculation accuracy. The theory is comprehensively verified by c omparison with spectroscopic data and, as a rule, agrees well with exp eriment. In the cases, where the discrepancy exceeds the calculation a ccuracy, it is indicative of new physical mechanisms.