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 à ).