Fachbereich Physik, Technische Universität Kaiserslautern, D-67653 Kaiserslautern, Germany.
J Chem Phys. 2011 Feb 14;134(6):064303. doi: 10.1063/1.3548874.
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(B)T)], and an activation energy E(a) 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. Foryś, 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) = k(1) exp[-E(a)∕E(0)]. 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 CH(3)Cl, CH(3)Br, CF(3)Cl, and CH(2)Cl(2) (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.
对于许多分子的缔合电子俘获（DEA），速率系数 k(T) 随温度 T 的升高而或多或少地升高（假设电子温度 T(e) 和分子温度 T(G) 处于热平衡，即 T = T(e) = T(G)）。这种升高通常用阿伦尼乌斯方程 k(T) = k(A) exp[-E(a)∕(k(B)T)]来建模，并且从对实验数据 k(T) 的拟合中得出活化能 E(a)。这种行为反映了阴离子在通向离解产物的路径上存在能垒。在最近的一篇论文中[J. Kopyra、J. Wnorowska、M. Foryś 和 I. Szamrej，Int. J. Mass Spectrom. 268, 60 (2007)]，有人提出，室温下 DEA 反应的速率系数大小表现出对活化能的指数依赖性，即 k(E(a); T ≈ 300 K) = k(1) exp[-E(a)∕E(0)]。对于具有高势垒的分子的最新实验数据[T. M. Miller、J. F. Friedman、L. C. Schaffer 和 A. A. Viggiano，J. Chem. Phys. 131, 084302 (2009)]与这种相关性是一致的。我们通过分析具有中间离解势垒的放热 DEA 过程的温度相关速率系数的 R 矩阵计算结果，研究了这种相关性的有效性和可能的起源。这些结果包括系统地改变势垒高度的模型体系的结果，以及针对 CH(3)Cl、CH(3)Br、CF(3)Cl 和 CH(2)Cl(2)（活化能高于 0.2 eV）的分子特异性计算结果，这些计算结果涉及到适当的分子参数。考虑到所考虑的分子类（卤代烷烃）的实验和理论结果的比较，支持了这样一种观点，即 k(T = 300 K)对活化能的指数依赖性反映了与 Franck-Condon 因子有关的一般现象，Franck-Condon 因子用于从初始中性振动能级直接进入离解的最终阴离子态。讨论了不适用所提出关系的情况。
