Temperature-Dependent Kinetics for the Reactions of Fe ( = 2-17) and FeNi ( + = 3-9) with O: 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 + O ( = 2-17) and for 300-500 K for FeNi + O ( + = 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. Measured rate constants for all three series (Fe, Ni, and FeN) vary over a relatively narrow range (1-5 × 10 cm s 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. The data are consistent with an anticorrelation between the electron binding energy and rate constant, previously noted in other systems. The Fe reaction produces a larger population of higher energy electrons than do the Ni reactions, with FeNi 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.