Impurity Electron Localization in Early-Transition-Metal-Doped Gold Clusters

We have performed X-ray absorption spectroscopy of size-selected, free, cationic, transition-metal-doped (Sc, Ti, V, Cr) gold clusters in a size range <i>n</i> = 1–9. The electronic configuration of the impurity atom was determined by modeling the X-ray absorption spectrum in a charge-transfer multiplet framework which makes it possible to quantify the amount of localization of the impurity 3d states. Depending on the dopant element and the host cluster size we find a wide variety in the behavior of local electronic structures. ScAu_<i>n</i>⁺ clusters show strong hybridization of the scandium 3d states with the host electronic states except for ScAu₁⁺ where we find a completely localized 3d electron. In TiAu_<i>n</i>⁺ clusters a pronounced odd–even alternation is present in the local electronic structure of the impurity atom. The 3d occupation number of the titanium dopant is approximately 2 and 1.6 electrons in odd and even-numbered clusters, respectively. In CrAu_<i>n</i>⁺ clusters the electronic structure of the dopant is governed by shell closure of the gold host which leads to almost unperturbed 3d states in CrAu_<i>n</i>⁺ (<i>n</i> = 2, 6, 8) and hybridization of the 3d states in CrAu_<i>n</i>⁺ (<i>n</i> = 1, 5, 7). Contrary to the other systems investigated the 3d occupation of 3.3 electrons in VAu_<i>n</i>⁺ clusters is independent of the cluster size. Only in special cases we find an integer number of localized 3d impurity electrons. Furthermore, in all cases the local electronic structure of the dopant does not strongly depend on the exact coordination of the dopant atom. This finding allows for a better understanding of the bonding beyond a simple shell model approach with its <i>ad hoc</i> assumption of integer numbers of delocalized impurity electrons