Quenching rate constants for NCl(b 1Σ+) and PCl(b 1Σ+) and radiative lifetimes of NCl(b 1Σ+), PCl(b 1Σ+) and PBr(b 1Σ+)

Abstract

NCl(b ¹Σ⁺), PCl(b ¹Σ⁺) and PBr(b ¹Σ⁺) molecules have been generated in a flow reactor by passing dilute flows of NFCl₂, PCl₃ and PBr₃ in He through a dc discharge, and the total rate constants for quenching of NCl(b ¹Σ⁺) and PCl(b ¹Σ⁺) by several diatomic and small polyatomic molecules have been measured at 300 K. Except for O₂, NO₂ and molecular halogens, the rate constants are in the 10^–13– 10^–14 cm³ molecule^–1 s^–1 range. The dependence of the rate constants on the properties of the reagent suggests that the dominant quenching mechanism for PCl(b) and NCl(b) is electronic-to-vibrational (E–V) energy transfer with the a ¹Δ state as the product, just as for the reactions of NF(b ¹Σ⁺) and PF(b ¹Σ⁺). The rate constants for quenching of NCl(b ¹Σ⁺) and PCl(b ¹Σ⁺) by O₂ are 1.0 × 10^–12 and ⩽ 10^–15 cm³ molecule^–1 s^–1, respectively, and are in accord with an electronic-to-electronic (E–E) energy transfer with the final states being O₂(b ¹Σ⁺)+ NCl(X ³Σ^–) or PCl(X ³Σ^–). Formation of PBr(b ¹Σ⁺) from the PCl(b ¹Σ⁺)+ Br₂ reaction was observed, and quenching of PCl(b ¹Σ⁺) by F₂, Cl₂ and Br₂ proceeds by a chemical mechanism with rate constants of 2.3 × 10^–12, 0.79 × 10^–12 and 16 × 10^–12 cm³ molecule^–1 s^–1, respectively. The radiative lifetimes for NCl(b ¹Σ⁺), PCl(b ¹Σ⁺) and PBr(b ¹Σ⁺) are 2.0 ± 0.4, 4.9 ± 0.8 and 0.8 ± 0.2 ms, respectively, as determined from the decay of the respective b ¹Σ⁺→ X ¹Σ^– emission intensities along the flow reactor.