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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.

Abstract. Reactivity of positively charged cobalt cluster ions (Co ], n = 2-22), produced by laser vaporization, with various gas samples (CH4, Nz, H2, C2H4, and C2Hz) were systematically investigated by using a fast-flow reactor. The reactivity of Co,, + with the various gas samples is qualitatively consistent with the adsorption rate of the gas to cobalt metal surfaces. Co. + highly reacts with C2H2 as characterized by the adsorption rate to metal surfaces, and it indicates no size dependence. In contrast, the reactions of Co.+ with the other gas samples indicate a similar cluster size dependence; at n = 4, 5, and 10-15, Co. + highly reacts. The difference can be explained by the amount of the activation energy for chemisorption reaction. Compared with neutral cobalt clusters, the size dependence is almost similar except for Co~ and Co~-. The reactivity enhancement of Co~ and Co; ~ indicates that the cobalt cluster ions are presumed to have an active site for chemisorption at n = 4 and 5, induced by the influence of positive charge.

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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.