Photoionization spectroscopy of nickel clusters: The effect of ammonia adsorption on ionization potentials

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Abstract
The photoionization efficiency spectra of nickel clusters containing adsorbed ammonia have been recorded near threshold. Ionization potentials (IPs) have been assigned for 97 selected Nin(NH3)m species, with n up to 56 atoms. We observe that nickel cluster IPs decrease upon ammonia adsorption, with the magnitude of the IP shifts varying linearly with the number of adsorbed molecules, in good agreement with a simple electrostatic model. Deviations from this behavior are observed for Ni5(NH3)m and Ni6(NH3)m clusters, in which adsorption of the first ammonia molecule produces a significantly larger IP drop than do subsequently adsorbed molecules. Such behavior may indicate the presence of unique adsorption sites on Ni5 and Ni6 preferentially occupied by the first ammonia molecule adsorbed onto these clusters, or alternatively, may indicate cluster rearrangement occurring as the result of chemisorption. Our results are discussed in light of studies of ammonia adsorption onto single crystal nickel surfaces.