Photoelectron spectroscopy of bimetallic aluminum cobalt cluster anions: Comparison of electronic structure and hydrogen chemisorption rates

The photoelectron spectra of small mass-selected aluminum-rich AlnCo- (n=8- 17) and cobalt-rich ConAlm- clusters (n = 6,8, 10; m = 1,2) are measured at photon energies of 3.49 eV with the aid of a magnetic bottle photoelectron spectrometer. The electronic structures of the bimetallic clusters are com pared with those of pure Al-n(-) and Co-n(-) clusters, which are measured u nder the same conditions. The threshold behavior (electron affinities and v ertical detachment energies) is analyzed, and the application of the electr onic shell model reveals a similar shell structure of Co-doped Al clusters with a single heteroatom. Additional heteroatom doping induces shell pertur bations. Compared to pure Al,, clusters, single-atom doped clusters show a hybridization of Al s, p and Co d orbitals resulting in a merging of spectr al features at low binding energies. The evolution of the electronic struct ure is compared with reported ionization potentials [Menezes and Knickelbei n, Chem. Phys. Lett. 183, 357 (1991); Z. Phys. D 26, 322 (1993)]. The hydro gen chemisorption behavior of neutral aluminum cobalt clusters [Nonose et a l., Chem. Phys. Lett. 164, 427 (1989)] shows a clear influence of Co d stat es at low binding energies in the case of Al,,Co, whereas for ConAlm cluste rs (n >m) the geometric effect becomes more dominant. (C) 2001 American Ins titute of Physics.