Near‐Infrared Spectrum of the First Excited State of Au(2) (+)

Au(2) (+) is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au(2) (+) ground state (X(2)Σ(g) (+)) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A(2)Σ(u) (+)). Herein, we present the vibrationally resolved electronic spectrum of this state for cold Ar‐tagged Au(2) (+) cations. This exceptionally low‐lying and well isolated A(2)Σ((u)) (+)←X(2)Σ((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 Au(2) (+), because the Au(2) (+)⋅⋅⋅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 σ(u) (*) orbital (HOMO‐1) into the bonding σ(g) orbital (SOMO), the Au−Au bond contracts substantially (by 0.1 Å).