Hydrogen forming reactions of thermal electrons in hydrogen halides: kinetics and thermodynamics

New competition kinetic studies of electron reactions in HCl and HBr in the 10¹⁸ to 10²⁰ molecule cm⁻³ concentration region are reported and shown to support the findings of earlier investigations. The capture of thermal electrons by (HX)₂ dimers is examined from a kinetic and thermodynamic point of view. The (HX)₂⁻* intermediate previously proposed by several groups is considered to be involved in both systems. Differences in the magnitudes and concentration dependences of the rates can be explained, if reaction 5b is fast [Formula: see text] in[Formula: see text]HBr but relatively slow in [Formula: see text] where [6a] and [6b] are the major product-forming reactions. Thermochemical data show [5b] to be strongly favoured energetically in[Formula: see text]HBr, and close to thermoneutral in HCl. Reaction 6b is energetically favourable in HCl. The present data require an autoionisation life-time of ∼10⁻¹² s for (HX)₂⁻*. From other considerations that of HX⁻* is expected to be [Formula: see text] This means that formation of (HX)₂⁻* must take place by collisions of electrons with HX molecules which are in effect already interacting. Production of stabilised HX⁻ from (HX)₂⁻* does not appear to contribute in HCl, but[Formula: see text]may occur in other HX Systems. The general implications of the mechanism for electron capture in HF and HX mixtures with other polar vapours are mentioned.