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

We present the cryogenic (26 K) IR spectra of selected [Ni_n(N₂)_m]⁺ (n = 5-20, m = 1 - m_max), which strongly reveal n- and m-dependent features in the N₂ stretching region, in conjunction with density functional theory modeling of some of these findings. The observed spectral features allow us to refine the kinetic classification [cf. J. Mohrbach, S. Dillinger, and G. Niedner-Schatteburg, J. Chem. Phys. 147, 184304 (2017)] and to define four classes of structure related surface adsorption behavior: Class (1) of Ni₆⁺, Ni₁₃⁺, and Ni₁₉⁺ are highly symmetrical clusters with all smooth surfaces of equally coordinated Ni atoms that entertain stepwise N₂ adsorption up to stoichiometric N₂:Ni_surface saturation. Class (2) of Ni₁₂⁺ and Ni₁₈⁺ are highly symmetrical clusters minus one. Their relaxed smooth surfaces reorganize by enhanced N₂ uptake toward some low coordinated Ni surface atoms with double N₂ occupation. Class (3) of Ni₅⁺ and Ni₇⁺ through Ni₁₁⁺ are small clusters of rough surfaces with low coordinated Ni surface atoms, and some reveal semi-internal Ni atoms of high next-neighbor coordination. Surface reorganization upon N₂ uptake turns rough into rough surface by Ni atom migration and turns octahedral based structures into pentagonal bipyramidal structures. Class (4) of Ni₁₄⁺ through Ni₁₇⁺ and Ni₂₀⁺ are large clusters with rough and smooth surface areas. They possess smooth icosahedral surfaces with some proximate capping atom(s) on one hemisphere of the icosahedron with the other one largely unaffected.