Structural assignment of small silver clusters

Johan van der Tol, Dewei Jia, Yejun Li, Valeriy Chernyy, Joost Bakker, Minh Nguyen, Ewald Janssens Silver clusters composed of a few atoms are very interesting for photography and redox catalysis. This is mainly because of their size dependent optical properties and a strong interplay between their geometric and electronic structure, which has a discrete density of states. Despite the wide interest in small silver clusters, there is so far, besides for Ag₃ and Ag₄ [1], no definitive (experimental) assignment of their geometry. Ion-mobility measurements on cationic silver clusters in the gas phase are available [2], but those measurements only provide cross sections. In addition, optical absorption spectra of Ag₄ to Ag₁₄ have been recorded, mapping their electronic excited states [3]. We have recorded the infrared multiphoton dissociation (IR-MPD) spectra of Ag₃⁺ to Ag₁₂⁺, by using a Free-electron laser in the far infrared (100-200 cm⁻¹ spectroscopic wavenumbers). The silver clusters were produced in a laser vaporization cluster source and tagged with weakly bound argon, with Arₙ up to n=4, which is released after resonant photon absorption. Comparison of calculated vibrational spectra for different structural isomers by density functional theory with the experimental IR-MPD spectra allows to determine the structures of the silver clusters. Meanwhile, the effect of Ar attachment on the smallest clusters( Ag₂⁺,Ag₃⁺, Ag₄⁺) was measured and calculated. [1] A. Fielicke et al. J. Phys. Chem. A 110, 8060 (2006) [2] P. Weis et al. Chem. Phys. Lett. 355, 355(2002) [3] Harb et al. J. Chem. Phys. 129, 194108 (2008)status: publishe