Energies and lifetimes (with respect to tunneling) for metastable f h [ ( ) 1 2 1 3 states o t e Henon-Heiles potential energy surface V x,y ~ 2 x -3 x + ~ y 2 + xy 2 ] have been computed quantum mechanically (via the method of complex scaling). This is a potential surface for which the classical dynamics is known to change from quasiperiodic at low energies to ergodiclike at higher energies. The rate constants (i.e. inverse lifetimes) for unimolecular decay as a function of energy, however, are seen to be well described by standard statistical theory (microcanomical transition state theory, RRKM plus tunneling) over the entire energy region, This is thus another example indicating that mode-specificity in unimolecular reaction dynamics is not determined solely by the 4uasiperiodic/ergodic character of the intramolecular mechanics.
I.Introduction.
Ion/molecule reaction products of cobalt cluster ions have been characterized using mass spectrometric techniques. Atomic and bare-metal cluster ions were desorbed from foils by particle bombardment within a high-pressure (0.1–0.2 Torr) ion source. Sputtered metal cluster ions react with O2 to produce abundant stoichiometric or nearly stoichiometric cobalt(II) oxide cluster ions. The positive cluster product ions consist of three types: oxygen-deficient [Co(CoO)x]+ clusters, oxygen-equivalent [(CoO)x]+ clusters, and (in less abundance) metal-deficient [(CoO)xO]+ clusters. Tandem mass spectrometry and collision spectroscopy provide structural information about the more abundant cobalt cluster product ions. A major collision-induced fragmentation pathway for the oxygen-equivalent [(CoO)x]+ clusters is the loss of a CoO moiety to form [(CoO)x−1]+ fragments. A major collision-induced fragmentation pathway for the oxygen-deficient [Co(CoO)x]+ clusters is the loss of a cobalt atom to yield [(CoO)x]+ fragments. Geometric structures of the cobalt/oxygen cluster ions were calculated using a Coulomb plus Born–Mayer pair-potential model. The oxygen-equivalent cluster structures were found to be ‘‘globular’’ cages, rings, or ladders. The oxygen-deficient cluster structures were found to be strained and ‘‘angular’’ with protruding cobalt atoms. The structures are discussed in terms of the observed collision-induced fragmentations. The fragmentations are rationalized using an ‘‘instantaneous’’ dissociation model of the collision-induced dissociation of the cluster ions. Preliminary trajectory calculations using classical dynamics support the use of this instantaneous dissociation model. The role of cluster ion structure in reactivity and collision-induced dissociation is discussed in terms of the experimental data and theoretical structures.
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