Reactions of size-selected Cu(n)(±) and Cu(n)O(m)(±) (n = 3-19, m ≤ 9) clusters with NO were investigated in the near-thermal energy region under single collision conditions using a tandem-type mass spectrometer with two ion-guided cells. Oxygen atoms preadsorbed on the cluster can significantly enhance the NO adsorption probability and cause additional reactions. NO adsorption is observed particularly for anionic copper cluster dioxides, Cu(n)O2(-) (n ≥ 8), followed by the release of a Cu atom from Cu(n)O2(-) (n = 8, 10, and 12), which suggests that NO adsorbs strongly, i.e., dissociatively on these clusters. Density functional theory calculations support that dissociative adsorption of NO occurs in the reaction of Cu8O2(-) under the present experimental conditions. On the other hand, NO oxidation proceeds in reactions of oxygen-rich cluster cations such as Cu4O3(+), Cu6O5(+), Cu9O7(+), and Cu11O8(+).

The reactions of methanol on a size-selected nickel cluster ion, Ni n + (n = 3−11), were investigated at collision energies less than 1.0 eV in a beam-gas geometry. Dominant reactions were methanol chemisorption, demethanation, and carbide formation. The absolute cross sections of these different reactions were measured and found to change dramatically with the cluster size; the demethanation proceeds preferentially on Ni4 +, the carbide formation on Ni7,8 +, and the chemisorption on Ni6 +. A kinematic model explains the size-dependent characteristics that the chemisorption proceeds efficiently if the barrier height between the physisorbed and the chemisorbed states is low, and the demethanation and the carbide formation proceed otherwise. The rate of the carbide formation depends sensitively on an Ni−Ni distance of Ni n + so that it proceeds only on Ni7,8 +.

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(n)O(2)(-). Both the cations and anions show the dominant formation of Cu(n-1)O(2)(±) in n ≤ 16 and Cu(n)O(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.

The collisional reactions of composition-selected cerium oxide cluster cations, CenOm(+) (n = 2-6; m ≤ 2n), with CO and NO have been investigated under single collision conditions using a tandem mass spectrometer. At near-thermal energy, oxidation of CO and NO is observed only for the stoichiometric clusters, CenO2n(+) (n = 3-5), and the cross sections for the NO oxidation are found to be larger than those for the CO oxidation. In addition, the collision-energy dependence of the reaction cross sections reveals that the CO oxidation has a small activation barrier, whereas the NO oxidation is a barrierless process. These experimental findings are supported by density functional theory calculations.

Vibrational predissociation spectra of NH4 +(NH3) n -1 (n = 6−9) were measured in the wavenumbers of 1045−1091 cm-1 by using a tandem mass spectrometer equipped with a serpentine octopole ion guide and a line-tunable cw CO2 laser. Each spectrum consists of an intense and broad peak and a weak one. The dissociation cross sections were in the order of 10-17−10-18 cm2. As n increases, the wavenumber of the intense and broad peak red-shifts gradually, and its intensity increases. The Hartree−Fock and the second-order Møller−Plesset calculations were performed to obtain the wavenumbers and the intensities of the optical absorption by these cluster ions with n = 2−8. The comparison of the experimental results with the calculations leads us to conclude that NH4 +(NH3) n -1 forms a shell structure and causes the absorption band associated with the collective ν2 vibration of NH3 to split.

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Collision-induced dissociation (CID) of acrylonitrile (AN: CHFCHCN) cluster cations and anions, (AN),= (n = 2-1 l), with an Ne or Kr atom was investigated in order to elucidate their structures. In the collision of (AN),-with a rare gas atom, [(AN),?-X]-(X = HCN, CH2CN) were produced along with (AN),,-( m I n). The size distributions of the product anions show that (AN),-is composed of a chemically bonded trimeric core anion solvated with chemically bonded AN trimer units and AN monomers. On the other hand, in a 1.0-eV collision of (AN),+ with a Kr atom, (AN)nr+ (rn 5 n ) were produced by evaporation of acrylonitrile molecules.

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