A kinetic study of the reactions of Ca+ ions with O3, O2, N2, CO2 and H2O

Abstract

The reactions between Ca⁺(4²S_1/2) and O₃, O₂, N₂, CO₂ and H₂O were studied using two techniques: the pulsed laser photo-dissociation at 193 nm of an organo-calcium vapour, followed by time-resolved laser-induced fluorescence spectroscopy of Ca⁺ at 393.37 nm (Ca⁺(4²P_3/2–4²S_1/2)); and the pulsed laser ablation at 532 nm of a calcite target in a fast flow tube, followed by mass spectrometric detection of Ca⁺. The rate coefficient for the reaction with O₃ is essentially independent of temperature, k(189–312 K) = (3.9 ± 1.2) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, and is about 35% of the Langevin capture frequency. One reason for this is that there is a lack of correlation between the reactant and product potential energy surfaces for near coplanar collisions. The recombination reactions of Ca⁺ with O₂, CO₂ and H₂O were found to be in the fall-off region over the experimental pressure range (1–80 Torr). The data were fitted by RRKM theory combined with quantum calculations on CaO₂⁺, Ca⁺·CO₂ and Ca⁺·H₂O, yielding the following results with He as third body when extrapolated from 10⁻³–10³ Torr and a temperature range of 100–1500 K. For Ca⁺ + O₂: log₁₀(k_rec,0/cm⁶ molecule⁻² s⁻¹) = −26.16 − 1.113log₁₀T − 0.056log₁₀²T, k_rec,∞ = 1.4 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, F_c = 0.56. For Ca⁺ + CO₂: log₁₀(k_rec,0/ cm⁶ molecule⁻² s⁻¹) = −27.94 + 2.204log₁₀T − 1.124log₁₀²T, k_rec,∞ = 3.5 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, F_c = 0.60. For Ca⁺ + H₂O: log₁₀(k_rec,0/ cm⁶ molecule⁻² s⁻¹) = −23.88 − 1.823log₁₀T − 0.063log₁₀²T, k_rec,∞ = 7.3 × 10⁻¹¹exp(830 J mol⁻¹/RT) cm³ molecule⁻¹ s⁻¹, F_c = 0.50 (F_c is the broadening factor). A classical trajectory analysis of the Ca⁺ + CO₂ reaction is then used to investigate the small high pressure limiting rate coefficient, which is significantly below the Langevin capture frequency. Finally, the implications of these results for calcium chemistry in the mesosphere are discussed.