Coinage metal clusters are of great importance for a wide range of scientific fields, ranging from microscopy to catalysis. Despite their clear fundamental and technological importance, the experimental structural determination of copper clusters has attracted little attention. We fill this gap by elucidating the structure of cationic copper clusters through IR photodissociation spectroscopy of Cu_n⁺-Ar complexes. Structures of Cu_n⁺ (n=3-10) are unambiguously assigned based on the comparison of experimental IR spectra in the 70-280 cm^-1 spectral range with spectra calculated using density functional theory (DFT). While Cu₃⁺ and Cu₄⁺ are planar, starting from n=5 Cu_n⁺ clusters adopt three-dimensional structures. Each successive cluster size is composed of its predecessor with a single atom adsorbed onto the face, giving evidence for a stepwise growth.

IR spectra of cationic copper clusters Cu_n⁺ (n = 4–7) complexed with hydrogen molecules are recorded via IR multiple-photon dissociation (IRMPD) spectroscopy. To this end, the copper clusters are generated via laser ablation and reacted with H₂ and D₂ in a flow-tube-type reaction channel. The complexes formed are irradiated using IR light provided by the free-electron laser for intracavity experiments (FELICE). The spectra are interpreted by making use of isotope-induced shifts of the vibrational bands and by comparing them to density functional theory calculated spectra for candidate structures. The structural candidates have been obtained from global sampling with the minima hopping method, and spectra are calculated at the semilocal (PBE) and hybrid (PBE0) functional level. The highest-quality spectra have been recorded for [5Cu, 2H/2D]⁺, and we find that the semilocal functional provides better agreement for the lowest-energy isomers. The interaction of hydrogen with the copper clusters strongly depends on their size. Binding energies are largest for Cu₅⁺, which goes hand in hand with the observed predominantly dissociative adsorption. Due to smaller binding energies for dissociated H₂ and D₂ for Cu₄⁺, also a significant amount of molecular adsorption is observed as to be expected according to the Evans–Polanyi principle. This is confirmed by transition-state calculations for Cu₄⁺ and Cu₅⁺, which show that hydrogen dissociation is not hindered by an endothermic reaction barrier for Cu₅⁺ and by a slightly endothermic barrier for Cu₄⁺. For Cu₆⁺ and Cu₇⁺, it was difficult to draw clear conclusions because the IR spectra could not be unambiguously assigned to structures.

Far-infrared multiple photon dissociation spectroscopy is used in combination with density functional theory calculations to determine the structures of isolated Au _n ⁺ ( n ≤ 9) clusters.