Probing the electronic structure and Au-C chemical bonding in AuC2- and AuC2 using high-resolution photoelectron spectroscopy

We report photoelectron spectroscopy (PES) and high-resolution PE imaging of AuC2- at a wide range of photon energies. The ground state of AuC2- is found to be linear (C-infinity v, (1)Sigma(+)) with a center dot center dot center dot 8 pi(4)4 delta(4)17 sigma(2)9 pi(4)18 sigma(2) valence configuration. Detachments from all the five valence orbitals of the ground state of AuC2- are observed at 193 nm. High-resolution PE images are obtained in the energy range from 830 to 330 nm, revealing complicated vibronic structures from electron detachment of the 18 sigma, 9 pi, and 17 sigma orbitals. Detachment from the 18 sigma orbital results in the (2)Sigma(+) ground state of neutral AuC2, which, however, is bent due to strong vibronic coupling with the nearby (2)Pi state from detachment of a 9 pi electron. The (2)Sigma(+)-(2)Pi vibronic and spin-orbit coupling results in complicated vibronic structures for the (2)Sigma(+) and (2)Pi(3/2) states with extensive bending excitations. The electron affinity of AuC2 is measured accurately to be 3.2192(7) eV with a ground state bending frequency of 195(6) cm(-1). The first excited state ((2)A') of AuC2, corresponding to the (2)Pi(3/2) state at the linear geometry, is only 0.0021 eV above the ground state ((2)A') and has a bending frequency of 207(6) cm(-1). The (2)Pi(1/2) state, 0.2291 eV above the ground state, is linear with little geometry change relative to the anion ground state. The detachment of the 17 sigma orbital also results in complicated vibronic structures, suggesting again a bent state due to possible vibronic coupling with the lower (2)Pi state. The spectrum at 193 nm shows the presence of a minor species with less than 2% intensity relative to the ground state of AuC2- . High-resolution data of the minor species reveal several vibrational progressions in the Au-C stretching mode, which are assigned to be from the metastable (3)Pi(2,1,0) spin-orbit excited states of AuC2- to the (2)Pi(3/2,1/2) spin-orbit states of neutral AuC2. The spin-orbit splittings of the (3)Pi and (2)Pi states are accurately measured at the linear geometry. The current study provides a wealth of electronic structure information about AuC2- and AuC2, which are ideal systems to investigate the strong Sigma-Pi and spin-orbit vibronic couplings. (C) 2014 AIP Publishing LLC.