Rate coefficients for quenching of Cl(2P1/2) by various atmospheric gases

Abstract

Rate coefficients for quenching of the upper spin–orbit state of chlorine Cl(²P_1/2) by a variety of collision partners have been measured at room temperature. The method used was laser flash photolysis of NOCl or ClONO₂ with resonance fluorescence detection of chlorine atoms using a strongly reversed resonance lamp for preferential detection of Cl(²P_1/2). The rate coefficients were (units of cm³ molecule^–1 s^–1): CF₄, (2.3 ± 0.3)× 10^–11; CF₂Cl₂, (3.3 ± 0.5)× 10^–10; CH₄, (2.2 ± 0.3)× 10^–11; ClONO₂, (1.7 ± 0.3)× 10^–10; O₂, (3.5 ± 0.5)× 10^–13; N₂, (5.0 ± 1.5)× 10^–15; He, (6.0 ± 1.0)× 10^–14; Ar, (3.0 ± 1.0)× 10^–16. These values are in good overall agreement with recent determinations which used IR laser detection of Cl(²P_1/2). Quenching by helium was found to be unusually fast, explaining in part discrepancies with earlier studies which used resonance absorption detection. The reaction with CH₄ was shown to proceed exclusively by quenching, and not to involve enhanced reactivity of Cl(²P_1/2) relative to the ground-state species. Additionally, the yield of Cl(²P_1/2) from the photolysis of chlorine nitrate was found to be 30% of the overall chlorine atom yield at 308 nm. The rate coefficient for the reaction of Cl(²P_3/2) with NOCl was measured to be (9.6 ± 1.5)× 10^–11 cm³ molecule^–1 s^–1.