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

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 sigma(+)(g)) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A(2)sigma(+)(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)sigma(+)((u))-X-2 sigma(+)((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 (similar to 0.4 eV) is much weaker than the Au-Au bond (similar to 2 eV). As a result of 6 s-5d excitation of an electron from the antibonding sigma(u)* orbital (HOMO-1) into the bonding sigma(g) orbital (SOMO), the Au-Au bond contracts substantially (by 0.1 angstrom).