Temperature-Dependent Kinetics for the Reactions of Fe n - ( n = 2-17) and Fe x Ni y - ( x + y = 3-9) with O 2 : Comparison of Pure and Mixed Metal Clusters with Relevance to Meteor Radio Afterglows and Surface Oxidation.

Rate constants and product branching fractions were measured from 300-600 K for Fe n - + O 2 ( n = 2-17) and for 300-500 K for Fe x Ni y - + O 2 ( x + y = 3-9) using a selected-ion flow tube (SIFT) apparatus. Rate constants for 46 species are reported. All rate constants increased with increasing temperature, and several were in excess of the Langevin-Gioumousis-Stevenson (LGS) capture rate at elevated temperatures. As with previously studied transition metal anion oxidation reactions, the collision limit is treated as the sum of the LGS limit along with a hard-sphere contribution, allowing for determination of activation energies. These values are compared to each other along with previous results for Ni n - . Measured rate constants for all three series (Fe n - , Ni n - , and Fe x N y - ) vary over a relatively narrow range (1-5 × 10 -10 cm 3 s -1 at 300 K) being at least 15% of the collision rate constant. All reaction rate constants increase with temperature, described by small activation energies of 0.5-4 kJ mol -1 . The data are consistent with an anticorrelation between the electron binding energy and rate constant, previously noted in other systems. The Fe n - reaction produces a larger population of higher energy electrons than do the Ni n - reactions, with Fe x Ni y - producing an intermediate amount. The results suggest that the overall rate constant is limited by a small energetic barrier located at a large internuclear distance where electrostatic forces dominate, causing the potentials to be similar across systems, while the product formation is determined by the shorter-range, valence portion of the potential, which varies widely between systems.