Study of electronically excited iodine atoms, I(52P½), in the presence of alkyl iodides

Abstract

Abstraction of an iodine atom from an alkyl iodide by an electronically excited iodine atom, I(5²P_½), is an exothermic process and thus may compete with collisional quenching at room temperature. I(5²P_½), generated by the flash photolysis of CF₃I, has been monitored by kinetic spectroscopy in absorption in the ultra-violet in the presence of seven alkyl iodides. The overall second-order rate constants for the removal of I(5²P_½) at ∼300°K were 1.7 × 10^–12, 1.9 × 10^–13, 2.0 × 10^–13, 2.0 × 10^–13, 2.9 × 10^–13, 2.9 × 10^–13 and 3.8 × 10^–13 cm³ molecule^–1 sec^–1 for CH₃I, C₂H₅I, n-C₃H₇I, i-C₃H₇I, n-C₄H₉I, i-C₄H₉I and t-C₄H₉I, respectively. Absorption spectroscopy in the vacuum ultra-violet following the flash photolysis of CH₃I itself, at low pressure, demonstrates that spin orbit relaxation dominates I atom abstraction, and it is proposed that electronic deactivation is the main process for all the iodides studied. For normal and isopropyl iodides, a difference in laser action from I(5²P_1/2) may be attributed to the relative yields of the atoms in the ²P_½ and ²P states on photodissociation rather than to any difference in the rates of electronic quenching of the excited atom by the iodides.