Collisional quenching of electronically excited chlorine atoms, Cl[3p 5(2 P 1/2)], 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(2P1/2)], has been studied by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet at λ= 136.34 nm {Cl[3p4 4s(2P3/2)]â†� Cl[3p5(2P01/2)]}. Cl(3 2P1/2) 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 2P1/2), 882 cm–1 above the 3 2P3/2 ground state, are reported (kQ/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 2P1/2) out of Boltzmann equilibrium for fundamental reasons discussed in this paper. General considerations of the chemistry of a Boltzmann system of Cl(3 2PJ) and the roles of the specific spin–orbit states, Cl(3 2P1/2) and Cl(3 2P3/2), 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 2P1/2) is derived, are also presented for most of the gases studied.