Probing cluster surface morphology by cryo kinetics of N₂ on cationic nickel clusters

We present the stepwise N₂ adsorption kinetics of size selected Ni_n⁺ (n = 5-20) clusters at 26 K as obtained by a hybrid tandem ion trap instrument. Pseudo-first-order kinetic fits confirm consecutive adsorption steps without evidence of cluster isomers and up to adsorption limits, which scale with the cluster size. The reaction rates for the initial N₂ adsorption increase smoothly with the cluster size and similar to hard sphere cluster modeling. The isothermal kinetics allow for the tentative elucidation of cluster surface morphologies and for their classification into highly symmetrical clusters with all smooth surfaces, small clusters with rough surfaces, and large clusters with partially rough and smooth surface areas. The parallel characterization of the vibrational spectroscopy of some cluster adsorbate complexes supports and refines the achieved conclusions and is published back to back with this contribution [S. Dillinger, J. Mohrbach, and G. Niedner-Schatteburg, J. Chem. Phys. 147, 184305 (2017)]. These two studies elucidate the adsorbate to cluster interaction, and they confirm and specify the sometimes considerable structural fluxionality of finite and curved metal surfaces in high detail. This work precedes further studies along the present lines of thought.