Photoelectric Observation of the Rate of Recombination of Iodine Atoms

The rate of the homogeneous three-body recombination of iodine atoms, I+I+M→I2+M, d[I2]/dt=k[I]2[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.