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

Au₂⁺ is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au₂⁺ ground state (X²Σ_g⁺) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A²Σ_u⁺). 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 σ_u^* orbital (HOMO-1) into the bonding σ_g orbital (SOMO), the Au−Au bond contracts substantially (by 0.1 Å).