Detection of Iodine Atoms by an Atomic Fluorescence Technique: Application to Study of Diffusion and Wall Recombination

An atomic fluorescence method has been used to detect iodine atoms in concentrations lower than 10¹⁰/cm³. The heart of the apparatus is a “solar‐blind” photomultiplier having practically no sensitivity to radiation of wavelengths longer than 2500 Å, but with good response in the region of the two strong iodine atomic resonance lines near 1800 Å. In combination with a small photolysis flashlamp, the system is used to study recombination of ground‐state ${(}^2{P}_{\text{3/2}})$ iodine atoms in pure I₂ vapor. At pressures below 0.2 torr the removal of atoms is seen to be predominantly a first‐order diffusion‐controlled process, characterized by a wall sticking probability of 0.040–0.065 and a binary diffusion cross section of 100 Ų. The reaction I+I₂→I₃ is found to be of minor significance in this study; a rough estimate indicates that fewer than one in 10⁴ I–I₂ collisions lead to the formation of I₃.