Electron attachment kinetics in flames. II. Dissociative attachment to HCl

This work comprises the first direct determination of the kinetics of Cl− formation in flames. The experiments involve mass spectrometric measurements of negative ion concentrations as functions of flow time in the burned gases of well-characterized laboratory test flames seeded with small quantities of alkali metals and chlorine compounds. The results are consistent with the two-body dissociative attachment mechanism e+HCl→Cl−+H with a rate coefficient well represented by the Arrhenius expression k=6×10−11 exp(−10 100/T) ml molecule−1 s−1 over the temperature range 1730 to 2475 K. The flames are operated at pressures of 100 and 760 Torr. Electron concentrations were measured using a microwave resonance cavity method and positive ion profiles (total charged species) were determined with electrostatic probes. H-atom concentration profiles required to take account of the reverse detachment process are well known for the 760 Torr flames; in the low pressure flames they were obtained using a mass spectrometric negative ion tracer technique involving the known ion–molecule chemistry of molybdenum additives and confirmed via optical absorption measurements of [OH]. Contributions to Cl− formation from three-body attachment processes are not observed.