The effect of increasing the excitation level of the sodium atom on the structure of the Na*Arn polyatomic exciplexes

In this work, the geometrical structures, the formation energies, and electronic states of the Na(ms)Ar-n polyatomic exciplexes with m = 3-6 and n = 2-5 are studied by using a quantum-classical method. The interaction potential between an electronically excited sodium atom and argon atoms are calculated by using a one-electron model involving electron-Ar and electron-Na+ pseudopotentials, in which the Hamiltonian is diagonalized at every optimization step in the Basin Hopping algorithm. The relationship between the position of the electronically excited levels and the cluster geometry is investigated as a function of the excitation level and of the spatial extension of the excited electron orbital. We show that the equilibrium structures of the ground state Na(3s)Ar-n and those of the electronically excited states Na(4s)Ar-n, Na(5s)Ar-n, and Na(6s)Ar-n are significantly different. As a result of the detailed examination of the relationships between the geometrical structure and density distribution of the Na valence electron of the Na*Ar-n with n = 2-5 polyatomic exciplexes, we can see that for the Na(4s)Ar-n polyatomic exciplexes, the two extreme geometries, neutral Na(3s)Ar-n and ionic Na+Arn compete. It appears that none of them is the actual one. For Na(5s)Ar-n and Na(6s)Ar-n the valence electron is very weakly bound to the ionic core and described by a more diffused orbital so that the geometry and the formation energies of this excited state called Rydberg states converge towards those of the ionic cores. (C) 2006 Elsevier Inc. All rights reserved.