Ab-initio study of optical response properties of nonstoichiometric lithium-hydride and sodium-fluoride clusters with one- and two-excess electrons

Structural and optical response properties of Li _n H _n-m and Na_nF _n-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 Na_nF _n-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, Li_nH _n-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 Li_nH _n-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, Na_nF _n-2 = 3-6) can be divided according to their optical and structural properties into cuboid “lattice” defect species (Na₄F _n , Na _n F₄) 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.