Structural assignment of small cationic silver clusters by far-infrared spectroscopy and DFT calculations

The structures of small cationic silver clusters Ag_n⁺ (n = 3-13) are investigated by comparing measured far-infrared multiple photon dissociation spectra of cluster-argon complexes with the calculated harmonic vibrational spectra of different low-energy structural isomers. A global structure search was carried out using the CALYPSO structure prediction method, after which isomers were locally optimized with the meta GGA functional TPSS. The obtained structures of the cationic silver clusters are mostly consistent with earlier ion mobility measurements and photodissociation spectroscopy studies for Ag_n⁺ (n = 3-11) and allowed excluding several structural isomers that were considered in those earlier studies, which illustrates the strength of combining multiple experimental techniques for conclusive structural identification. The growth pattern of the cationic silver clusters is discussed and differences with other cationic coinage metal clusters are highlighted.