Adsorption reaction of polar organic molecules on Si_n ⁺ cluster ions

We have examined chemical reactions of small silicon cluster ions Si _n⁺ for n = 7-16 with polar organic molecules M (M = CH₃CN, CD₃OD, C₂H₅CN, and C ₂H₅OH). The intensities of the adsorption products Si _nM_m⁺ for m = 1 and 2 were investigated as a function of n. We found for all polar molecules that the relative intensity of Si_nM⁺ to the unreacted Si_n⁺ is smaller for n = 11, 13, and 14, that is, the adsorption reactivity is smaller for these n than others. It was also commonly observed that the SigM ₂⁺ ion are more intense than neighboring n. We discussed the relationship of the reactivity with the geometrical structures and the stabilities of the bare Si_n⁺ ions and adsorbed Si _n(CH₃CN)_m⁺ ions, from theoretical calculations based on density functional theory.