The Optical Spectrum of Au2

• Published in: Angewandte Chemie International Edition 2020-11, Vol.59 (48), p.21403-21408
• Main Authors: Förstel, Marko, Pollow, Kai Mario, Saroukh, Karim, Najib, Este Ainun, Mitric, Roland, Dopfer, Otto
• Format: Article
• Language: English
• Subjects: Catalytic activity; Cations; Communication; Communications; Coupling (molecular); Electron states; Electronic structure; electronic structure calculations; gold; Mathematical analysis; Molecular ions; optical spectroscopy; Photodissociation; Spectroscopy; Spectrum analysis
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.202011337

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. The optical spectrum of the prototypical diatomic Au2+ cation is presented in high quality. The measured spectrum of this seemingly simple H2+‐like cation cannot be explained by standard TD‐DFT methods but requires relativistic multireference treatment with spin‐orbit coupling.