Hot-Atom Reactions in the Photolysis of Deuterium Chloride at 1849 Å

Abstract

The importance of hot deuterium atoms in photolyses of DCl–lCl₂ mixtures at 1849 Å has been demonstrated, and the kinetics of this system have been described. Photodecomposition of DCl was followed by mass‐spectrometric analysis of the D₂ formed after successive intervals of time. The observed decreases in the initial rate of D₂ formation upon addition of CO₂, and inert gases CO₂, Xe, and CF₄ may be readily interpreted in terms of a hot‐atom mechanism but cannot be made compatible with a completely thermal mechanism. Rate coefficient ratios for the hot‐atom reactions, $$\mathrm{D}\mathrm{*+}\mathrm{DCl}\to {\displaystyle {lim}^2}{\mathrm{D}}_2+\mathrm{Cl},$$ $$\mathrm{D}\mathrm{*+}\mathrm{M}\to {\displaystyle {lim}^3}\mathrm{D}+\mathrm{M},$$ $$\mathrm{D}\mathrm{*+}{\mathrm{Cl}}_2\to {\displaystyle {lim}^6}\mathrm{DCl}+\mathrm{Cl},$$ were found to be $k_6{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_2{\text{\hspace{0.17em}≈\hspace{0.17em}6.5, k}}_3^{\mathrm{DCl}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_2{\text{\hspace{0.17em}=\hspace{0.17em}0.65, k}}_3^{{\mathrm{CO}}_2}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_2{\text{\hspace{0.17em}=\hspace{0.17em}1.15, k}}_3^{{\mathrm{CF}}_4}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_2\text{\hspace{0.17em}≈\hspace{0.17em}1.15,}\mathrm{and}{k}_3^{\mathrm{Xe}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_2\text{\hspace{0.17em}<\hspace{0.17em}0.1}$ . The ratio $k_6{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_2$ increased from 6.5 in the absence of inert gas to 9.2 when [CO₂]/[DCl] = 1.87. This ratio increases to ≈ 300 for a thermal atom distribution at 300°K. Comparisons of thermalizing efficiencies for several molecules suggest that inelastic collisions are important in thermalizing hot atoms produced photochemically at 2.1 eV.