Atomic resonance fluorescence spectrometry for rate constants of rapid bimolecular reactions. Part 1.—Reactions O + NO2, Cl + ClNO, Br + ClNO

Abstract

Atomic resonance fluorescence spectrometry in the vacuum ultra-violet, in conjunction with a discharge-flow system, has been used to investigate ground state O³P_J and halogen (Cl, Br, I)²P atoms. The method is extremely sensitive and the following lower limits of concentrations detectable were obtained, in atom cm^–3 : [O ³P_J] 4 × 10¹⁰, [Cl ²P] 5 × 10¹⁰, [Br ²P] 1.5 × 10¹⁰, [I ²P] 8 × 10⁹. In all cases studied, the intensity of fluorescence varied in direct proportion to atom concentration at low atom concentrations (< 10¹² atom cm^–3). Factors which might interfere with measurements of atom concentrations by resonance fluorescence—electronic quenching, dissociative excitation of molecular halogen, excitation of molecular resonance series—were unimportant under the conditions used.

The use of the method for the measurement of very rapid atom + molecule bimolecular reactions approaching the hard-sphere bimolecular collision frequency was demonstrated for the three atom titration reactions (1), (2) and (3): O ³P_J+ NO₂ [graphic omitted] NO + O₂(1), Cl ²P+ ClNO [graphic omitted] NO + Cl₂(2), Br ²P+ ClNO [graphic omitted] NO + BrCl.(3) Detailed study of the kinetics of these reactions at 298 K was carried out for the first time. Pseudo first-order kinetic analysis ([atom][molecule]), with measurements of -d[atom]/dt by resonance fluorescence, were used to determine the rate constants k₁, k₂ and k₃(cm³ molecule^–1 s^–1): k₁=(6.1 ± 0.6)× 10^–12, k₂=(3.0 ± 0.5)× 10^–11, k₃=(1.0 ± 0.2)× 10^–11 at 298 K. The value for k₁ is in good agreement with previous work. The agreement between the present and an earlier, indirect measurement of k₂ is less satisfactory; possible reasons for this discrepancy are discussed. k₃ has not been reported previously.