The electronic structure of the Au2+ cation is essential for understanding its catalytic activity. We present the optical spectrum of mass‐selected Au2+ measured via photodissociation spectroscopy. Two vibrationally resolved band systems are observed in the 290–450 nm range (at ca. 440 and ca. 325 nm), which both exhibit rather irregular structure indicative of strong vibronic and spin‐orbit coupling. The experimental spectra are compared to high‐level quantum‐chemical calculations at the CASSCF‐MRCI level including spin‐orbit coupling. The results demonstrate that the understanding of the electronic structure of this simple, seemingly H2+‐like diatomic molecular ion strictly requires multireference and relativistic treatment including spin‐orbit effects. The calculations reveal that multiple electronic states contribute to each respective band system. It is shown that popular DFT methods completely fail to describe the complex vibronic pattern of this fundamental diatomic cation.

Infrared multiple-photon dissociation spectroscopy reveals the nature of nitrous oxide binding to metal clusters.

The optical spectrum of Si 2 + is presented. The two electronic band systems observed near 430 and 270 nm correspond to the two lowest optically allowed transitions of Si 2The spectra were measured via photodissociation spectroscopy of mass-selected ions at the level of vibrational resolution, and the determined spectroscopic constants provide detailed information about the geometric and electronic structure, establishing molecular constants of this fundamental diatomic cation that enable astrophysical detection on, for example, hot rocky super-Earth-like exoplanets.

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Au2+ is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au2+ ground state (X2Σg+) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A2Σu+). Herein, we present the vibrationally resolved electronic spectrum of this state for cold Ar‐tagged Au2+ cations. This exceptionally low‐lying and well isolated A2Σ(u)+←X2Σ(g)+ transition occurs in the near‐infrared range. The observed band origin (5738 cm−1, 1742.9 nm, 0.711 eV) and harmonic Au−Au and Au−Ar stretch frequencies (201 and 133 cm−1) agree surprisingly well with those predicted by standard time‐dependent density functional theory calculations. The linearly bonded Ar tag has little impact on either the geometric or electronic structure of Au2+, because the Au2+⋅⋅⋅Ar bond (∼0.4 eV) is much weaker than the Au−Au bond (∼2 eV). As a result of 6 s←5d excitation of an electron from the antibonding σu* orbital (HOMO‐1) into the bonding σg orbital (SOMO), the Au−Au bond contracts substantially (by 0.1 Å).