THE GEOMETRIC STRUCTURES AND OPTICAL-RESPONSE PROPERTIES OF SMALL NANMG CLUSTERS

The influence of an alkaline-earth atom on the structural and electron ic properties of alkali metal clusters is reported. In the lowest ener gy structures of Na5Mg and Na6Mg, the Mg atom takes the place of one o f the Na atoms in the topology of the energetically low lying structur es of the corresponding Na(n+1) clusters. In the case of Na7Mg and Na8 Mg, those structures containing deformed pentagonal bipyramids as subu nits (with the Mg atom at the apical position capped by one or two add itional Na atoms) are very close in energy to structures in which the Mg atom is located centrally, with larger coordination number. The opt ically allowed transitions and oscillator strengths for several struct ures of Na6Mg and Na8Mg clusters calculated using the multiconfigurati onal linear response method (MCLR) are compared with those obtained fr om the random phase approximation (RPA). Both methods give rise to sim ilar dominant features, but the inclusion of higher order excitations is found to influence the fine structure of the spectra which might be of importance for comparison with the experimental data. The spectros copic patterns for both clusters with 8 and 10 valence electrons are s trongly dependent on the position of the nuclei. Structures with the M g atom in the central position give rise to a very different distribut ion of intense transitions compared to those with Mg atom at the perip heral position.