Kinetic investigation of the reaction between Na + O2+ M by time-resolved atomic resonance absorption spectroscopy

Abstract

A kinetic investigation has been carried out on ground-state sodium atoms, Na(32S), generated by the ultraviolet pulsed irradiation of NaI vapour at the temperatures T= 724 and 844 K, and monitored by time-resolved atomic resonance absorption in the “single-shot” mode at λ= 589 nm (32PJâ†� 32S). In the presence of the gases He, N2 and CO2 alone, the atoms exhibit diffusional decay with removal at the walls of the vessel. The variation of the decay of Na (32S) as a function of the pressure of these gases, coupled with the use of the “long-time” solution of the diffusion equation for a cylinder, yields the following average diffusion coeffcients (corrected to s.t.p.): DNa–He= 0.25, DNa–N2= 0.24 ± 0.3 and DNa–CO2= 0.30 cm2 s–1. The third-order reactions between Na + O2+ M (M = He, N2 and CO2) were investigated at T= 724 and 844 K, the rate constants indicating no significant temperature variation. The following third-order absolute rate constants for these processes for the range T= 724–844 K are reported: k(M = He)=(6 ± 1)× 10–31 cm6 molecule–2 s–1, k(M = N2)=(1.0 ± 0.24)× 10–30 cm6 molecule–2 s–1, k(M = CO2)= 2 × 10–30 cm6 molecule–2 s–1, the datum for CO2 being reliable to within a factor of two. The data are compared with those from analogous measurements for other metal atom reactions of the type X + O2+ M and are found to be of comparable magnitude. By contrast, the present results are ca. three orders of magnitude greater than those reported hitherto from flame measurements.