Collisional quenching of OH (A 2Σ+, v′ = 0) by N2, O2 and CO2 between 204 and 294 K. Implications for atmospheric measurements of OH by laser-induced fluorescence

Abstract

The temperature dependence of the collisional quenching of OH (A ² Σ ⁺ , v′ = 0) has been studied in a discharge-flow system between 204 and 294 K for the collision gases N ₂ , O ₂ and CO ₂ . Quenching cross-sections were observed to increase with decreasing temperature, consistent with attractive forces dominating the collision process. For N ₂ , the cross-section increases from 4.3 ± 0.1 Å ² at 294 ± 1 K to 6.8 ± 0.2 Å ² at 204 ± 1 K, for O ₂ from 18.6 ± 0.6 Å ² (294 K) to 23.5 ± 0.7 Å ² (204 K) and for CO ₂ from 62.8 ± 1.9 Å ² (292 K) to 75.6 ± 2.3 Å ² (204 K). The results are in good agreement with previous determinations, but extend the range to lower temperatures, and are consistent with data obtained at much higher temperatures. The results are also consistent with the cross-section for quenching by air determined at ultra-low temperatures (ca. 25 K) within a supersonic free-jet expansion. A collision complex model incorporating the rotational dependence of quenching using an exponential energy gap law was able to reproduce the data for O ₂ and CO ₂ over the temperature range studied. The experimental results were used to calculate the effective cross-section for quenching of OH (A ² Σ ⁺ , v′=0) by air in the range 200–300 K, covering conditions found both in the stratosphere and within the free-jet expansion of instruments used to measure tropospheric OH by low-pressure laser-induced fluorescence. The fluorescence quantum yield was calculated to change by ca. 25% in this range because of changes in quenching rate, directly affecting instrument calibrations and hence their sensitivity for OH detection.