Experimental and theoretical study of the temperature and pressure dependences of the recombination reactions O + NO2(+M) → NO3(+M) and NO2 + NO3(+M) → N2O5(+M)

Abstract

The recombination reactions O + NO₂(+M) → NO₃(+M) and NO₂ + NO₃(+M) → N₂O₅(+M) were studied at temperatures of 300 and 400 K, and at pressures of the bath gas M = N₂ between 1 and 900 bar. Oxygen atoms were generated by laser flash photolysis of N₂O at 193 nm, NO₃ radicals were monitored by light absorption at 578 nm. The measured fall-off curve of the reaction O + NO₂(+M) → NO₃(+M) could be well represented by limiting low pressure rate constants k_3,0 = (1.3 ± 0.3) × 10⁻³¹ (T/300 K)^−1.5 [N₂] cm⁶ molecule⁻² s⁻¹, limiting high pressure rate constants k_3,∞ = (2.3 ± 0.2) × 10⁻¹¹ (T/300 K)^0.24 cm molecule⁻¹ s⁻¹, and fall-off broadening factors of F_c = 0.71 exp(−T/1700 K). The derived results are consistent with earlier relative rate measurements. Theoretical modeling of k_3,0, F_c and k_3,∞ led to consistency with the experimental data. There is strong evidence that, besides the electronic ground state NO₃(²A′), the first excited electronic state NO₃(²E′) also contributes to the observed recombination reaction. The measured fall-off curve of the reaction NO₂ + NO₃(+M) → N₂O₅(+M) was represented by limiting low pressure rate constants k_5,0 = 3.6 × 10⁻³⁰(T/300 K)^−5.0 [N₂] cm⁶ molecule⁻² s⁻¹, limiting high pressure rate constants k_5,∞ = (1.9 ± 0.3) × 10⁻¹²(T/300 K)^0.2 cm³ molecule⁻¹ s⁻¹ and F_c = 0.38 exp(−T/4900 K). A theoretical analysis of these values is also presented.