Ab-initio study of optical response properties of nonstoichiometric lithium-hydride and sodium-fluoride clusters

Structural and optical response properties of LinHn-m and NanFn-m (n = 2-6, m = 1,2) clusters containing one- and two-excess electrons are studied using ab-initio methods accounting for electron correlation. The common feature of the optical response obtained for the most stable structures of NanFn-1 (n = 2-6) clusters is the appearance of a dominant intense transition in the infrared regime independently whether the single excess electron is localized at the cuboid corner vacancy (surface F-center) or at the external atom attached to the filled cuboid. In contrast, LinHn-1 (n = 2-6) clusters exhibit substantially different spectroscopic patterns with respect to halides also for the cases with the common structural properties. Optical response features of LinHn-2 (n = 3-6) clusters with two-excess electrons are characterized by dominant transitions in the visible regime reflecting segregation in "metallic" and ionic parts. In contrast, NanFn-2 (n = 3-6) can be divided according to their optical and structural properties into cuboid "lattice" defect species (Na4F2, Na6F4) and segregated metallic-ionic systems. For the former, the intense transitions occur in the infrared-visible, and for the latter only in the visible regime. It will be shown that the calculated absorption patterns are excellent fingerprints of structural and bonding properties. (C) 1997 Elsevier Science B.V.