Optical Absorption of Small Palladium-Doped Gold Clusters

The effect of Pd doping on the structure and optical absorption of small cationic gold clusters is investigated by a combined photodissociation spectroscopy and time-dependent density functional theory study of Au_n ⁺Ar_p and PdAu_{n -1} ⁺Ar_p (n = 4,5; p = 0,1). While pure Au clusters are planar, the Pd-doped clusters are 3D. UV-visible absorption is studied in the 2.0-4.7 eV photon energy range, allowing the observation of previously unreported absorption bands for Au₄ ⁺ and Au₄ ⁺Ar. The oscillator strength of the optical transitions is dramatically reduced upon incorporating a Pd atom in Au₄ ⁺ and Au₄ ⁺Ar, while this effect is less pronounced for Au₅ ⁺Ar. Analysis of the electron density transfer shows a different influence of Pd with size. While Pd has a formal negative charge in Au₃Pd⁺, in Au₄Pd⁺ most of the charge is attracted by the highly coordinated central Au atom, leaving Pd positively charged, also affecting the induced structural changes. In addition, orbital analysis of the optical transitions is carried out in order to identify the levels involved in the optical absorption of the pure Au and Pd doped clusters. A reduction of the s density near the Fermi energy, induced by Pd doping, causes a quenching of optical absorption.