Ab initio study of structural and optical response properties of excess-electron lithium-hydride and sodium-fluoride clusters

Structural and optical response properties of stoichiometric small LinHn and NanFn and non-stoichiometric LinHn and NanFm (n - m = 1, 2, 3, 8) clusters containing single and multiple excess electrons are studied using ab initio methods accounting for electron correlation. We show that calculated absorption patterns are excellent fingerprints of structural and bonding properties. The optical response of NanFn-1 (n = 2-6) clusters is characterized by the common feature that is the appearance of a dominant intense transition in the infrared regime, independently whether the excess electron is localized at the cuboid corner vacancy 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 nuclearities with analogous structures. NanFn-2 (n = 3-6) clusters containing two excess electrons can be divided according to their optical and structural properties into cuboid ''lattice'' defect species (Na4F2, Na6F4) and segregated metallic-ionic systems (Na3F, Na5F3). For the former, intense transitions are found in the infrared-visible region, and for the latter only in the visible regime. LinHn-2 (n = 3-6) clusters exhibit common optical response features with dominant transitions in the visible regime which is characteristic for segregated metallic-ionic systems. Ground and excited state properties of above systems are in good agreement with available experimental data. Comparison with properties found for pure alkali-metal clusters allow one to investigate metallization and localization processes in finite systems.