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

Abstract

We present a theoretical study of Na(2) (+) solvation in an argon matrix Ar(n) 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 Na(2) (+)Ar(n) clusters. For n<or=5, the lowest energy isomers are obtained by aggregation of Ar atoms at one single extremity of Na(2) (+), leading to moderate perturbation of the optical transition. For 6<or=n<or=15, the Ar atoms aggregate at both extremities. This structural change is associated with a strong blueshift of the first optical transition (X (2)Sigma(g) (+)-->A (2)Sigma(u) (+)), which reveals the confinement of the excited A (2)Sigma(u) (+) state. The Na(2) (+) energy spectrum is so strongly perturbed that the A (2)Sigma(u) (+) state becomes higher than the B (2)Pi(u) (+) 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 Na(2) (+) molecular axis. The optical transitions vary smoothly with n and the A (2)Sigma(u) (+) and B (2)Pi(u) states are no longer inverted, though the first optical transition remains strongly blueshifted.