The structures of cationic gold clusters probed by far-infrared spectroscopy.

Determining the precise structures of small gold clusters is an essential step towards understanding their chemical and physical properties. Due to the relativistic nature of gold, its clusters remain planar (2D) up to appreciable sizes. Ion mobility experiments have suggested that positively charged gold clusters adopt three-dimensional (3D) structures from n = 8 onward. Computations predict, depending on the level of theory, 2D or 3D structures as putative energy-minimum for n = 8. In this work, far-infrared multiple photon dissociation spectroscopy, using Ar as tagging element, is combined with density-functional theory calculations to determine the structures of Au (n≤ 9) clusters formed by laser ablation. While the Au frameworks in AuAr and AuAr complexes are confirmed to be planar and that in AuAr three-dimensional, we demonstrate the coexistence of 3D and planar AuAr (m = 1-3) isomers. Thus, it is revealed that at finite temperatures, the formal 2D to 3D transition takes place at n = 8 but is not sharp.