Photoelectric Observation of the Rate of Recombination of Iodine Atoms

The rate of the homogeneous three‐body recombination of iodine atoms, $\mathrm{I}+\mathrm{I}+\mathrm{M}\to {\mathrm{I}}_2+\mathrm{M}\mathrm{,\hspace{0.17em}d[}{\mathrm{I}}_2\mathrm{]/dt=k[}\mathrm{I}{]}^2[\mathrm{M}]$ , is measured. A short intense pulse of light from a flash lamp dissociates 1–9 percent of the ca 10¹⁸ iodine molecules in a 200‐cc cell, and the subsequent recombination of iodine atoms is followed by fast photoelectric techniques. The measured values of k at room temperature are 4.2(±0.4)×10⁹ liter² moles⁻² sec⁻¹ (argon), 58(±4)×10⁹ (neopentane), 65(±6)×10⁹ (pentane). The value for neopentane is the same at 200°C. The large values of the recombination rate constants indicate that an important recombining process consists of a ``sticky'' collision between an I atom and an M molecule or atom, leading to the formation of a complex IM, which reacts with a second I atom. The lifetimes of the collision complexes are estimated. The rates of the reverse process, the dissociation of I₂ by collision with M, are calculated; the pre‐exponential factors are extraordinarily large. The extinction coefficients of gaseous iodine have been remeasured.