Crystal field effects on the reactivity of aluminum-copper cluster anions

The limits and useful modifications of the jellium model are of great interest in understanding the properties of metallic clusters, especially involving bimetallic systems. We have measured the relative reactivity of ${\text{CuAl}}_{n}^{\ensuremath{-}}$ clusters $(n=11--34)$ with ${\text{O}}_{2}$. An odd-even alternation is observed that is in accordance with spin-dependant etching, and ${\text{CuAl}}_{22}^{\ensuremath{-}}$ is observed as a ``magic peak.'' The etching resistance of ${\text{CuAl}}_{22}^{\ensuremath{-}}$ is explained by an unusually large splitting of the $2{\text{D}}^{10}$ subshell that occurs because of a geometric distortion of the cluster that may also be understood as a crystal field splitting of the superatomic orbitals.