Kinetic study of electronically excited atoms, I(52P½) in the presence of olefines

Abstract

The decay of electronically excited iodine atoms, I(5²P_½), in the presence of a number of olefines has been studied by kinetic absorption spectroscopy in the ultra-violet and the vacuum ultra-violet. The mode of removal is dominated by collisionally induced spin orbit relaxation to the I(5²P) ground state rather than by chemical reaction. The second-order rate constants for electronic quenching by the molecules ethylene, propylene, 1-butene, trans-butene-2, cis-butene-2, isobutene, tetramethylethylene, C₂F₄ and C₂F₃H are 7.0 × 10^–14, 1.6 × 10^–13, 1.6 × 10^–13, 2.1 × 10^–13, 2.2 × 10^–13, 3.0 × 10^–13, 6.3 × 10^–13, 3.7 × 10^–15 and 5.3 × 10^–14 cm³ molecule^–1 s^–1, respectively. For the hydrocarbon olefines, these rate constants correlate with ionization potential, excitation energy of the π—π^* transition and heat of hydrogenation. Thus, on collision, a charge-transfer interaction with the π electrons of the olefine is involved, leading to relaxation.