Communication: Vibrational spectroscopy of Au4 from high resolution photoelectron imaging.

High resolution photoelectron spectroscopy of Au4 (-) is reported using a new photoelectron imaging apparatus. A broad vibrational progression is resolved for the detachment transition from the ground electronic state of the Y-shaped Au4 (-) to that of the Y-shaped Au4 neutral (C2v, (1)A1) in the ν2 vibrational mode with a harmonic frequency of 171(7) cm(-1) and an anharmonicity of ∼0.5 cm(-1). In addition, two low frequency modes with weak Franck-Condon factors are observed: the v3 mode with a frequency of 97(7) cm(-1) and the v6 mode with a frequency of 17(7) cm(-1). An accurate electron affinity of 2.7098(6) eV is obtained for the Y-shaped Au4 neutral cluster. The current study shows that very low frequency vibrational modes can be resolved for size-selected clusters using high resolution photoelectron imaging, providing valuable additional experimental information for cluster structure determination.