A Saturated LIF Study on the High Pressure Limiting Rate Constant of the Reaction CN + NO + M → NCNO + M between 200 and 600 K

The reaction CN + NO + M ↔ NCNO + M was studied in the bath gas helium at temperatures between 200 and 600 K and in the pressure range between 1 and 100 bar. CN radicals were generated by laser flash photolysis of BrCN at 193 nm in the presence of NO and high helium pressures. The concentration of CN radicals was detected by recording their non-resonant fluorescence yield at 420 nm after delayed excitation in the (0, 1)-band of the X²∑⁺ → B²∑⁺ transition near 387.8 nm. Thermal second order rate constants were extracted from the fluorescence-yield time profiles under pseudo-first order conditions. We obtained access to wide parts of the falloff range and constructed complete falloff curves using the Troe formalism. We expressed the low and high pressure limiting rate constants as k_1,0 = [He] (2.1±0.7)x 10^-30 (T/300 K)^-(2.1±0.3) cm⁶ s^-1 and as k_1,∞ = (5.4±1.4)x 10^-11 (T/300 K)(0.0±0.2) cm³ s^-1 and gave a broadening factor F_C(He) = exp(-T/590 K) + exp(-947 K/T). In separate experiments we monitored the non-resonant fluorescence yield of CN at 418.5 nm in the presence of NO after delayed excitation in the (1, 2)-band of the B²∑⁺ X²∑⁺ transition near 386.6 nm. The rate constant for collisional deactivation of vibrationally excited CN radicals in the electronic ground state CN(ν=1) + NO →CN(ν=0) + NO is strongly related to the high pressure limiting recombination rate constant. We determined its rate constant to be k₂ = (5.3±1.3)x 10^-11 (T/300 K)(-0.2±0.2) cm³ s^-1 in excellent agreement with our high pressure limiting recombination rate constant.