Level-Structure and Magnetic Properties from One-Electron Atoms to Clusters with Delocalized Electronic Orbitals: Shell Models for Alkali Trimers

Abstract

The triatomic alkali–metal clusters we investigate here are still accessible via molecular-orbital-based ab initio computation, but do already show some features characteristic of extended many-electron systems. Experimental observation of both the low-spin multiplicity (doublet) and the high-spin multiplicity (quartet) presents us with two species with very different binding mechanisms (covalent vs. van der Waals). The Complete Active Space Self Consistent Field method in combination with Multi Reference Rayleigh Schrödinger Perturbation Theory of second order is applied to the high-spin (quartet) states of all possible homonuclear and heteronuclear trimers of K and Rb. We calculate the first few electronically excited states and compare the results with the doublet system of K3 and Rb3. The classical shell model for metal trimers is briefly reviewed in relation to the low-spin configuration and compared with a new model we introduce to rationalize the significantly different level structure we observe for the high-spin. This structure turns out to be related to the eigenstates of the harmonic oscillator, a feature known from the description of single-particle states in quantum dots.