Kinetic study of the absolute rate constant for the reaction of caesium and N 2O by time-resolved atomic resonance absorption spectroscopy

Abstract

We present a kinetic study of the reaction of ground state atomic caesium with N 2O by direct spectroscopic monitoring in the time domain. Cs(6 2S 1 2 ) was generated by the pulsed irradiation of caesium chloride vapour in the presence of N 2O and excess helium in a static system and monitored by time-resolved atomic resonance absorption spectroscopy in the “single-shot mode” using the resolved Rydberg doublet at λ = 455.5 nm (Cs(7p( 2P 3 2 )) ← Cs(6s( 2S 1 2 )). The spectroscopic source was a newly constructed high intensity, high current hollow cathode source. Photoelectric signals at the resonance wavelength representing the decay of Cs(6 2S 1 2 ) were captured, digitised and stored in a transient recorder and transferred to a microcomputer for kinetic analysis. The decay profiles were employed to characterise the absolute second-order rate constant k 1 for the reaction Cs + N 2O → CsO + N 2 across the limited temperature range 847 – 865 K, for which we report the average result of k 1 = (1.9 ± 0.3) × 10 −11 cm 3 per molecule s −1. This is compared with those from previous kinetic studies of the reactions between lithium, sodium, potassium and rubidium + N 2O derived from direct spectroscopic monitoring in the time domain, and indicate that the reaction with N 2O may be employed for titration of atomic caesium in a flow system and for generating CsO in known concentrations for subsequent investigation. An estimate of the diffusion coefficient for caesium in helium of D 12(CsHe) = about 0.2 cm 2 s −1 at s.t.p. is reported. Finally, the result is briefly considered in terms of the symmetry of the potential surfaces involved.