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)

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 k3,0 = (1.3 ± 0.3) × 10−31 (T/300 K)−1.5 [N2] cm6 molecule−2 s−1, limiting high pressure rate constants k3,∞ = (2.3 ± 0.2) × 10−11 (T/300 K)0.24 cm molecule−1 s−1, and fall-off broadening factors of Fc = 0.71 exp(−T/1700 K). The derived results are consistent with earlier relative rate measurements. Theoretical modeling of k3,0, Fc and k3,∞ 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 k5,0 = 3.6 × 10−30(T/300 K)−5.0 [N2] cm6 molecule−2 s−1, limiting high pressure rate constants k5,∞ = (1.9 ± 0.3) × 10−12(T/300 K)0.2 cm3 molecule−1 s−1 and Fc = 0.38 exp(−T/4900 K). A theoretical analysis of these values is also presented.