The kinetics of M_n^- + O₂ (M = Al (n=1-17), Cr (n=1-15), Ni (n=1-14), Co (n=1-12), V (n=1-15) ) were measured from 300 to 600 K using a selected-ion flow tube apparatus. The Al_n^-anions, long believed to have stable values of n (e.g. 13 and 23) resistant to reaction with O₂, all reacted and surprising showed positive temperature dependences well-described by Arrhenius equation behavior. Much of the reactiviity was attributed to electron production. The activation energies where linear with electron binding energy for n > 8 and extrapolated to the surface value, showing that the mechanism is similar. Historically, even clusters were found to react faster than odd ones which had been attributed to spin constraints in the odd species. Instead, we found the pre-exponential factors were larger for the odd species and that a simple counting of the number of accessible curves in the even clusters explained the results; spin had the opposite effect to what had been previously postulated. Extending these results to transition metals showed much faster reactivity and in fact a number of the clusters had rate constants that either extrapolated to or exceeded the Langevin-Gioumousis-Stevenson capture rate constant. Application of a capture theory model accounting for the finite size of the clusters reproduces the cluster size-dependent trends in reactivity. Assumption that reactivity is controlled by an energetic barrier early in the reaction coordinate, similar but smaller to that found for Al, quantitatively reproduces the experimental kinetics. A proposed mechanism involving various oxidation states of O₂ indicates that the barrier arises from a long-range charge transfer as has been shown previously for Al surface oxidation. Extrapolations of the EBA vs E_a to the surface values again shows that the mechanisms are similar, indicating that gas-phase reactions of anionic metal clusters are appropriate model systems for surface oxidation. The reactions mostly proceed by production of high energy electrons as evidence by the inability of SF₆ to attach to the electrons. These reactions then appear to be models for the production of the high energy electrons needed to explain meteor radio afterglows caused by the ablating particles.

Juergen Troe

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