The structure of nickel–iron clusters probed by adsorption of molecular nitrogen

Abstract

The isomeric structures of small nickel-rich nickel–iron alloy clusters are probed via the adsorption of molecular nitrogen on their surfaces. A determination of nitrogen saturation levels allows the location of the iron atoms to be established. Experiments on pure iron clusters demonstrate very little adsorption of molecular nitrogen under conditions where pure nickel clusters show saturated coverages. This low adsorption on iron appears to carry over to the alloy clusters. If the iron atoms are on the surface of a cluster, nitrogen saturation levels are lower than for pure nickel clusters, while if they are inside, coverage levels are the same. Results are presented for clusters containing 13, 19, 20, 23, 26, and 48–53 atoms. They show that alloy clusters in the size ranges where nickel clusters tend to have icosahedral or polyicosahedral packing are most stable with the iron atoms located inside. Unlike other clusters, Ni 26 shows a clear change in geometrical structure when a single nickel atom is replaced by an iron atom. Evidence is also presented for the formation of metastable isomeric configurations in which iron atoms are located outside. These isomers convert to the more stable ones in the experimental time scale of a few ms. The effect of the nitrogen overlayer on the conversion process is discussed.