Collisional quenching of electronically excited chlorine atoms, Cl[3p5(2P1/2)], by atmospheric gases studied by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet

Abstract

The collisional quenching of electronically excited chlorine atoms, Cl[3p⁵(²P_1/2)], has been studied by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet at λ= 136.34 nm {Cl[3p⁴ 4s(²P_3/2)]â†� Cl[3p⁵(²P⁰_1/2)]}. Cl(3 ²P_1/2) was generated by the repetitive pulsed irradiation of CCl₄ in the presence of excess helium buffer gas. The excited atom was then monitored photoelectrically using signal averaging in the short-time domain before the onset of Boltzmann equilibrium. The following absolute second-order rate constants for the removal of Cl(3 ²P_1/2), 882 cm^–1 above the 3 ²P_3/2 ground state, are reported (k_Q/cm³ molecules^–1 s^–1, 300 K, errors 1 σ) for a wide range of gases of atmospheric interest: N₂, (6.3 ± 1.0)× 10^–13; CO₂, <5 × 10^–13; O₂, (2.3 ± 0.3)× 10^–11; N₂O, (3.7 ± 0.6)× 10^–13; H₂, <6 × 10^–13; H₂O, (2.6 ± 0.5)× 10^–12; HCl, (1.1 ± 0.1)× 10^–12; CH₄, (3.9 ± 0.8)× 10^–12; CO, ca. 6 × 10^–12. These rate data are compared with the collisional-quenching rate constants for the other gases that we have reported previously using this technique which is, at present, the only practical method for monitoring Cl(3 ²P_1/2) out of Boltzmann equilibrium for fundamental reasons discussed in this paper. General considerations of the chemistry of a Boltzmann system of Cl(3 ²P_J) and the roles of the specific spin–orbit states, Cl(3 ²P_1/2) and Cl(3 ²P_3/2), are presented with special reference to CH₄ and the atmospheric gases in general. Relative rate data for the quenching of the transient precursor, generated from the photolysis of CCl₄ and from which Cl(3 ²P_1/2) is derived, are also presented for most of the gases studied.