Impurity doping of a simple metal cluster: An ab initio study of Li₉F

The ground-state properties of the doped alkali-metal cluster, Li₉F, have been investigated using extended basis Hartree-Fock calculations. Six isomers were found. The excited states of the isomers were modeled via configuration-interaction singles (CIS). Differences between the optical response of the isomers with an external fluorine and with an internal fluorine were found. Comparisons are also made between the CIS results for the isomers of Li₉F and the homogeneous clusters, Li₈ and Li₉⁺. The ground-state studies show that the metallic Li₈ core is resistant to substantial modification; the added LiF modifies the geometry and the symmetry in all six isomers, but the frontier molecular orbital structure is only slightly changed from the parent Li₈ cluster. All six isomers found are substantially noncentrosymmetric. The reduced symmetry permits optical absorption at many more frequencies than can be seen in the more highly symmetric Li₈ or Li₉⁺ parents, but the overall spectral pattern is very similar.