Solvation of Na2 + in Arn clusters. I. Structures and spectroscopic properties

Abstract : We present a theoretical study of Na2 + solvation in an argon matrix Arn for n=1 to a few tens. We use a model based on an explicit description of valence electron interaction with Na+ and Ar cores by means of core polarization pseudopotential. The electronic structure determination is thus reduced to a one-electron problem, which can be handled efficiently.We investigate the ground state geometry and related optical absorption of Na2 +Arn clusters. For n≤5, the lowest energy isomers are obtained by aggregation of Ar atoms at one single extremity of Na2 +, leading to moderate perturbation of the optical transition. For 6≤n≤15, the Ar atoms aggregate at both extremities. This structural change is associated with a strong blueshift of the first optical transition (X 2Σg + →A 2Σu +), which reveals the confinement of the excited A 2Σu + state. The Na2 + energy spectrum is so strongly perturbed that the A 2Σu + state becomes higher than the B 2πu + states. The closure of the first solvation shell is observed at n=17. Above this size, the second solvation shell develops. Its structure is dominated by a pentagonal organization around the Na2 + molecular axis. The optical transitions vary smoothly with n and the A 2Σu + and B 2πu states are no longer inverted, though the first optical transition remains strongly blueshifted.