Comparison of adsorption probabilities of O ₂ and CO on copper cluster cations and anions

Reactions of size-selected copper cluster cations and anions, Cu _n ^±, with O ₂ and CO have been systematically investigated under single collision conditions by using a tandem-mass spectrometer. In the reactions of Cu _n ^± (n = 3-25) with O ₂, oxidation of the cluster is prominently observed with and without releasing Cu atoms at the collision energy of 0.2 eV. The reactivity of Cu _n ⁺ is governed to some extent by the electronic shell structure; the relatively small reaction cross sections observed at n = 9 and 21 correspond to the electronic shell closings, and those at odd sizes in n ≤ 16 match with the clusters having no unpaired electron. On the other hand, the reactivity of Cu _n ^- exhibits no remarkable decrease by the electronic shell closings and the even-numbered electrons. These behaviors may be due to an influence of the electron detachment of the reaction intermediate, Cu _nO ₂ ^-. Both the cations and anions show the dominant formation of Cu _n-1O ₂ ^± in n ≤ 16 and Cu _nO ₂ ^± in n ≤ 17 in the experimental time window. By contrast, Cu _n ^- (n = 3-11) do not react with CO at the collision energy of 0.2 eV, while Cu _n ⁺ (n = 3-19) adsorb CO though the cross sections are relatively small. The difference in the reactivity between the charge states can be understood in terms of the frontier orbitals of the Cu cluster and O ₂ or CO.