Evidence for Thermal Hydrogen-Atom Reactions in Irradiated Gaseous HCl

The addition of SF6, which is known to have a high cross section for reaction with thermal electrons, reduces the hydrogen yield from irradiated gaseous HCl to G(H2) = 3.9. The yield of hydrogen from pure HCl is G(H2) = 8.0 and the reduction of 4.1 G units achieved by the SF6 is within experimental error equal to Gionization for the system. Both Cl2 and Br2 cause a greater reduction to a residual or ``unscavengable'' hydrogen yield, GH″, of 2.2±0.2 G units. In the case of Cl2 this value is within experimental error independent of temperature over the range −79° to 23°C. The addition of Cl2 to SF6—HCl mixtures of a composition such that virtually all electrons are scavenged by SF6 causes a reduction from the SF6 plateau to the same residual yield observed with Cl2 and Br2 alone. This reduction may be attributed to a competition between Cl2 and HCl for thermal H atoms, which are formed in processes not involving thermal electrons as direct precursors. A competition kinetic study gave the following values of kH+HCl/kH+Cl2: 8.7±0.5×10−3 at 23°C; 7.5±0.5×10−3 at 1°C; and 1.6±0.5×10−3 at −77°C. The values at 23° and 1°C are in good agreement with recent experimental values of this ratio obtained by other methods. Possible origins of the thermal hydrogen atoms and the residual hydrogen yield are discussed.