Resonant photoelectron spectroscopy of Au2(-) via a Feshbach state using high-resolution photoelectron imaging.

Photodetachment cross sections are measured across the detachment threshold of Au2(-) between 1.90 and 2.02 eV using a tunable laser. In addition to obtaining a more accurate electron affinity for Au2 (1.9393 ± 0.0003 eV), we observe eight resonances above the detachment threshold, corresponding to excitations from the vibrational levels of the Au2(-) ground state (X(2)Σu(+)) to those of a metastable excited state of Au2(-) (or Feshbach resonances) at an excitation energy of 1.9717 ± 0.0003 eV and a vibrational frequency of 129.1 ± 1.5 cm(-1). High-resolution photoelectron spectra of Au2(-) are obtained using photoelectron imaging to follow the autodetachment processes by tuning the detachment laser to all the eight Feshbach resonances. We observe significant non-Franck-Condon behaviors in the resonant photoelectron spectra due to autodetachment from a given vibrational level of the Feshbach state to selective vibrational levels of the neutral final state. Using the spectroscopic data for the ground states of Au2(-) (X(2)Σu(+)) and Au2(X(1)Σg(+)), we estimate an equilibrium bond distance of 2.53 ± 0.02 Å for the Feshbach state of Au2(-) by simulating the Franck-Condon factors for the resonant excitation and autodetachment processes.