Kinetics of NCl(aΔ and b¹Σ⁺) Generation: The Cl + N₃ Rate Constant, the NCl(a¹Δ) Product Branching Fraction, and Quenching of NCl(a¹Δ) by F and Cl Atoms

Abstract

The quenching rate constants for NCl(a1Δ) by F and Cl atoms have been measured at room temperature to be (2.2 ± 0.7) × 10-11 and (1.0 + 1.0/−0.5) × 10-12 cm3 s-1, respectively, by adding F and Cl atoms to a flow reactor containing NCl(a1Δ). With knowledge of these quenching rate constants, the kinetics for the formation of NCl(a1Δ) from the Cl + N3 reaction could be investigated in the F/Cl/HN3 reaction system. The reduction in NF(a1Δ) yield from adding Cl atoms to the reactor containing F and HN3 and the relative NF(a1Δ) and NCl(a1Δ) yields for known concentrations of F and Cl atoms in this reaction system favor a total Cl + N3 rate constant of 3 ± 1 × 10-11 cm3 s-1 with a branching fraction for NCl(a1Δ) formation of ≳ 0.5. The branching fraction was deduced from comparing the relative intensities of the NCl(a−X) and NF(a−X) transitions using a lower limit to the NCl(a) radiative lifetime of 2 s. The direct formation of NCl(b1Σ+) from Cl + N3 is a minor channel; however, NCl(b1Σ+) is formed by bimolecular energy pooling of NCl(a1Δ) molecules with a rate constant of ≈1.5 × 10-13 cm3 s-1 and by energy transfer between NCl(a1Δ) and HF(v ≥ 2). The bimolecular energy-pooling process is a small fraction of the total bimolecular self-destruction rate for NCl(a1Δ).