Comparison of Adsorption Probabilities of O2 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, CunO2-. Both the cations and anions show the dominant formation of Cun-1O2 +/- in n <= 16 and CunO2 +/- 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.