Probing the 2D to 3D structural transition in gold cluster anions using argon tagging.

Different physisorption properties by 2D and 3D isomers of Au_{n};{-} clusters are observed and used to probe the 2D to 3D structural transition. Strong Ar clustering occurs on planar Au_{n};{-} and the planar faces of the pyramidal Au20-. An abrupt change of Ar clustering at Au12- confirms the 2D to 3D structural transition at this size, where both isomers coexist. The minor 2D isomer can be titrated out by Ar to produce a clean 3D-Au12- beam and beams of Au_{12}Ar_{m};{-} with enhanced 2D isomers. Using the Ar titration and tagging, isomer-specific photoelectron spectra for the 2D and 3D Au12- are obtained.