On the relation between the activation energy for electron attachment reactions and the size of their thermal rate coefficients

Authors

H. Hotop, Technische Universität KaiserslauternFollow
M.-W. Ruf, Technische Universität Kaiserslautern
J. Kopyra, University in Siedlce
T. M. Miller, Air Force Research Laboratory
I. I. Fabrikant, University of Nebraska-LincolnFollow

Date of this Version

2011

Citation

THE JOURNAL OF CHEMICAL PHYSICS 134, 064303 (2011)

Comments

© 2011 American Institute of Physics

doi:10.1063/1.3548874

Abstract

Rate coefficients k(T) for dissociative electron attachment (DEA) to molecules in many cases exhibit a more or less strong rise with increasing temperature T (the electron temperature T_e and the molecular temperature T_G are assumed to be in thermal equilibrium, i.e., T = T_e = T_G). This rise is frequently modeled by the Arrhenius equation k(T) = k_A exp[−E_a/(k_BT)], and an activation energy Ea is deduced from fits to the experimental data k(T). This behavior reflects the presence of an energy barrier for the anion on its path to the dissociated products. In a recent paper [J. Kopyra, J. Wnorowska, M. Fory´s, and I. Szamrej, Int. J. Mass Spectrom. 268, 60 (2007)] it was suggested that the size of the rate coefficients for DEA reactions at room temperature exhibits an exponential dependence on the activation energy, i.e., k(E_a; T ≈ 300 K) = k₁ exp[−E_a/E₀]. More recent experimental data for molecules with high barriers [T. M. Miller, J. F. Friedman, L. C. Schaffer, and A. A. Viggiano, J. Chem. Phys. 131, 084302 (2009)] are compatible with such a correlation. We investigate the validity and the possible origin of this dependence by analyzing the results of R-matrix calculations for temperature-dependent rate coefficients of exothermic DEA processes with intermediate barrier toward dissociation. These include results for model systems with systematically varied barrier height as well as results of molecule-specific calculations for CH₃Cl, CH₃Br, CF₃Cl, and CH₂Cl₂ (activation energies above 0.2 eV) involving appropriate molecular parameters. A comparison of the experimental and theoretical results for the considered class of molecules (halogenated alkanes) supports the idea that the exponential dependence of k(T = 300 K) on the activation energy reflects a general phenomenon associated with Franck–Condon factors for getting from the initial neutral vibrational levels to the dissociating final anion state in a direct DEA process. Cases are discussed for which the proposed relation does not apply.