Atomic resonance fluorescence spectrometry for rate constants of rapid bimolecular reactions. Part 4.—Chlorine atom fluorescence 4s2,4P–3p52P

Abstract

Atomic fluorescence in the vacuum ultraviolet arising from excited multiplet states of Cl(4s⁴P_J, 4s²P_J and 4s′²D_J) was generated in a flow system by, (a) irradiation of ground state Cl atoms with a chlorine atom resonance lamp, (b) photolysis of Cl₂ with a He resonance lamp and (c) impact of Cl₂ with He 2s³S₁ metastable atoms. For all eight transitions (to the ground 3p⁵²P_J states) studied in (a), the intensity of Cl atom resonance fluorescence varied in direct proportion with ground state Cl atom concentration, N, with sufficiently low values of N; for instance, at 138.0 nm this value of N was ⩽ 1 × 10¹² cm^–3; and at 134.7 nm, N was ⩽ 3 × 10¹¹ cm^–3. The most sensitive transitions for resonance fluorescence detection of Cl under the conditions used were the 138.0 nm and the 134.7 nm lines.

Atomic fluorescence intensities were measured, and used to derive certain branching ratios for Einstein coefficients for spontaneous emission involving the Cl multiplet transitions, A^λ_ki: A_ki^139.7/A_ki^138.0=(0.13 ± 0.02); A_ki^136.3/A_ki^134.7=(0.14 ± 0.02) and A_ki^135.2/A_ki^133.6=(1.7 ± 0.3).

A resonance fluorescence technique of high sensitivity for kinetic studies in a flow system (∼200 N m^–2 total pressure) is described, giving detection of ground state Cl concentrations down to 3 × 10⁹ cm^–3 with signal-to-noise of unity. It was used to measure the rate constant for the extremely rapid reaction of Cl with Br₂, Cl + Br₂→ BrCl + Br, k²⁹⁸=(1.9 ± 0.2)× 10^–10 cm³ molecule^–1 s^–1.