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.