Au2H- was recognized and confirmed as a minor contamination to typical photoelectron spectra of Au-2(-), produced by laser vaporization of a pure Au target using an ultrahigh purity helium carrier gas. The hydrogen source was shown to be from trace H impurities present in the bulk gold target. Carefully designed experiments using H-2- and D-2-seeded helium carrier gas were used to study the electronic structure of Au2H- and Au2D- using photoelectron spectroscopy and density functional calculations. Well-resolved photoelectron spectra with vibrational resolution were obtained for Au2H- and Au2D-. Two isomers were observed both experimentally and theoretically. The ground state of Au2H- turned out to be linear with a terminal H atom [Au-Au-H](-) ((1)A(1),C-infinityv), whereas a linear [Au-H-Au](-) ((1)A(1),D-infinityh) structure with a bridging H atom was found to be a minor isomer 0.6 eV higher in energy. Calculated electron detachment energies for both isomers agree well with the experimental spectra, confirming their existence in the cluster beam. The observation and confirmation of H impurity in pure gold clusters and the 3.44 A Au-Au distance in the [Au-H-Au](-) isomer presented in the current work provide indirect experimental evidence that the anomalous 3.6 A Au-Au distances observed in gold nanowires is due to an "invisible" hydrogen impurity atom. (C) 2004 American Institute of Physics.