Mechanism of Radiolysis of Gaseous HBr

Abstract

The ion‐pair yield for the decomposition of gaseous HBr by 120‐kV x rays has been determined from measurements of hydrogen yields and saturation ionization currents in an ionization chamber. At HBr pressures in the range 200 to 305 torr, $M_{\mathrm{‐HBr}}\mathrm{\hspace{0.17em}/\hspace{0.17em}N}\mathrm{is}\text{4.8\hspace{0.17em}±\hspace{0.17em}0.1.}$ Up to one‐half of this yield may be due to the excitation of HBr molecules to neutral excited states, and their subsequent dissociation to Br and H atoms. Under the conditions employed, all H atoms are expected to undergo Reaction (7): $$\mathrm{H}\mathrm{\hspace{0.17em}+\hspace{0.17em}}\mathrm{HBr}\to {\mathrm{H}}_2\mathrm{+Br.}$$ (7) A study of the effects of SF₆ indicated that the yield of scavengeable thermal electrons, which form hydrogen, is 3.1 per 100 eV. The results of competition experiments with this scavenger were incompatible with electron‐positive‐ion combination and the dissociative capture reaction, $$\mathrm{e\hspace{0.17em}+\hspace{0.17em}}\mathrm{HBr}\to \mathrm{H}\mathrm{\hspace{0.17em}+\hspace{0.17em}}{\mathrm{Br}}^{-}$$ , (3) as sources of hydrogen molecules or atoms. They may be explained in terms of thermal electron capture reactions which are second order in HBr. Physical evidence for at least one such process, viz., $$\text{e\hspace{0.17em}+\hspace{0.17em}2}\mathrm{HBr}\to \mathrm{H}\mathrm{\hspace{0.17em}+\hspace{0.17em}}\mathrm{Br}\mathrm{\hspace{0.17em}—\hspace{0.17em}}\mathrm{H}\mathrm{\hspace{0.17em}—\hspace{0.17em}}{\mathrm{Br}}^{-}$$ , (14a) has recently been obtained.