Flash Photolysis of H2O2 Vapor in the Presence of D2, Ar, and H218O

Abstract

The flash photolysis of H₂O₂ by itself, and in the presence of D₂, Ar, and H₂¹⁸O, was studied by the technique of kinetic absorption spectroscopy. The only observable primary products were OH(²Π_½) and OH(²Π${}_{\frac{3}{2}}$ ). These ground‐state levels were populated in a ratio of ∼1:1, respectively, conforming to what is observed with the halogens. The contribution of Reaction (a) to the primary process was shown in a direct way to be no more than 20% of that of (b). Reaction (c) followed by (d) was shown to make no significant contribution to the OH concentration during the time of our experiments (<200μ sec): $${\mathrm{H}}_2{\mathrm{O}}_2(\mathrm{singlet}\mathrm{)\to }{\displaystyle {lim}_{\mathrm{h\nu }}}{\mathrm{H}}_2\mathrm{O}{(}^1{A}_1\mathrm{)+}\mathrm{O}{(}^1\mathrm{D),}$$ $$\text{→2}\mathrm{OH}{(}^2\mathrm{\Pi ),}$$ $$\to {\mathrm{H}}_2\mathrm{O}{(}^3{A}_1\mathrm{)+}\mathrm{O}{(}^3\mathrm{P),}$$ $$\mathrm{O}{(}^3\mathrm{P)+}{\mathrm{H}}_2{\mathrm{O}}_2\mathrm{\hspace{0.17em}(}\mathrm{singlet}\mathrm{)\to }\mathrm{OH}{(}^2\mathrm{\Pi )+}{\mathrm{HO}}_2\mathrm{\hspace{0.17em}(}\mathrm{doublet}\mathrm{),}$$ $$\mathrm{OH}{(}^2\mathrm{\Pi )+}{\mathrm{H}}_2{\mathrm{O}}_2\mathrm{\hspace{0.17em}(}\mathrm{singlet}\mathrm{)\to }{\mathrm{H}}_2\mathrm{O}{(}^1{A}_1\mathrm{)+}{\mathrm{HO}}_2\mathrm{\hspace{0.17em}(}\mathrm{doublet}\mathrm{).}$$ Thus, this technique is a convenient one for directly observing reactions of OH. An upper limit to the rate constant of (e) at room temperature was determined to be 5.8±1.8×10⁻¹⁴ cm³ molecule⁻¹·sec⁻¹. Conditions for generating vibrationally hot molecules in the primary photolytic step are discussed.