The Optical Spectrum of Au₂⁺

Abstract

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.

Spectroscopy and photochemistry of copper nitrate clusters

“…Ultraviolet/visible/near-infrared (UV/vis/NIR) photodissociation spectroscopy is a very powerful tool to characterize the complex electronic structure of transition metal complexes and clusters in the gas phase. [44][45][46][47][48][49][50][51][52][53][54] We recently investigated photochemical hydrogen evolution from hydrated magnesium, 55,56 the effect of salt environments on the reactions and photolysis of organic substances, [57][58][59] as well as the evolution of the hydration environment of a single electron 60 or a carbon dioxide radical anion 61 using action spectroscopy.…”

Atomic Cluster Au10+ Is a Strong Broadband Midinfrared Chromophore

“…Using ion mobility mass spectrometry Kappes and coworkers revealed remarkable planar ground state structures for n≤7 (Aun + ) and n≤11 for Aunclusters [11][12][13]. Far infrared action spectroscopy of Aun + (n≤9) recently confirmed these structures [14][15][16] and visible photodissociation spectroscopy has been used to probe structure around the 2D to 3D transition [17] and other sizes [18][19][20][21][22][23][24][25][26][27][28][29]. Various studies of ligated clusters also show evidence of this large geometrical change [30][31][32].…”

Near‐Infrared Spectrum of the First Excited State of Au2+

“…In this respect, Au 10 + exhibits a particularly low and broad transition centered at around 0.55 eV, which extends down into the vibrational domain of the ground electronic state. For Au 2 + , calculations predict an optically active and well isolated A 2 Σ u + state around 0.8 eV above the X 2 Σ g + ground state arising from 6 s←5d excitation of an electron from the antibonding σ u * orbital (HOMO‐1) into the bonding σ g (s) orbital (SOMO), as shown in Figure 1 [3a,6] . As no other (bright) states are nearby, coupling to other states is expected to be weak, at least near the potential minimum of the A state.…”

“…The band origin (0 0 ) of the à 2 Σ + ← Σ + transition (correlating with A 2 Σ u + ←X 2 Σ g + for bare Au 2 + ) is observed at 5738 cm −1 (1743 nm, 0.711 eV). A long progression of up to ten quanta in the ν 3 mode with a harmonic frequency of ω 3 =201(1) cm −1 ( ν 3 =199 cm −1 ) is assigned to the Au−Au stretch vibration, based on comparison with the higher excited states of Au 2 + and the frequency of neutral Au 2 ( ν 3 =190 cm −1 in the A state) [3a,4] . The long ν 3 progression peaking at n =3 indicates a substantial change in the Au−Au bond length upon electronic excitation.…”

“…Concerning Au 2 + , we have so far characterized higher excited states in the 300–700 nm range, giving rise to two complex band systems near 440 and 325 nm, which both exhibit rather irregular vibronic structure due to strong coupling of multiple electronic states occurring in the same energy range [3a] . This congested vibronic structure could only be explained by sophisticated multi‐reference calculations including spin–orbit coupling and relativistic corrections [3a] . Clearly, standard time‐dependent density functional theory (TD‐DFT) calculations completely fail to reproduce the observed spectral pattern [3a] .…”