The Electronic Structure of the Superatom Au3+

At the heart of large gold clusters lies Au3+, a key three-ring structure that plays a fundamental role in many gold assemblies. Studying this smallest unit provides valuable insight into the photocatalytic mechanisms of larger gold systems. This study presents the first high-resolution, vibrationally resolved optical spectra of mass-selected Au3+ clusters obtained through photodissociation spectroscopy. The spectra, spanning 2.7–5.0 eV, reveal five band systems with complex and irregular structures. To interpret these spectra, we employ ab initio calculations at the CASSCF level, incorporating spin–orbit coupling to characterize the electronic structure of Au3+. These calculations reveal that the observed spectral features arise from the interplay of closely spaced excited states dominated by d-orbital excitations, a small s–d orbital gap, and significant vibronic and spin–orbit coupling. Our findings emphasize that accurate characterization of the excited states of Au3+, a seemingly simple triatomic molecule with a valence configuration and orbital structure similar to that of the strontium atom, requires sophisticated multireference calculations. Simplified theoretical methods, such as density functional theory (DFT) and those utilizing small HOMO–LUMO active spaces, fail to capture the strong multireference effects driven by d-orbital contributions, underscoring the complexity of excited-state interactions in gold clusters.