A discharge stopped-flow technique for time-resolved studies of slow radical reactions in the gas phase

A stopped-flow technique is described for measuring overall rate constants in the range 10-15-10-19 cm3 molecule-1 s-1 for reactions of a radical in the gas phase. The technique has been applied specifically to the study of the nitrate radical; a description is given of the generation of the radical (involving a microwave discharge) and of its detection by optical absorption. The detection system allows the recording of decays in NO3 signal (absorption at lambda =662 nm) over a period of up to 60 s after isolation in the absorption cell and with a detectability for NO3 down to 2*1011 molecule cm-3 for five co-additions. The reaction of NO3 with ethylene is used as an example to show the applicability of the technique to a slow reaction, the measured rate constant being (1.7+or-0.5)*10-16 cm3 molecule-1 s-1.