A 9 GHz free-space microwave interferometer was used to measure the electron density decline over the range 1011-1010 cm−3 in the afterglow of a pulsed argon discharge in a 12·7 cm diameter cylindrical tube (characteristic diffusion length Λ=2·46 cm). In the range 0·14-0·61 torr the results are interpreted in terms of the ion conversion mechanism Ar++2Ar->Ar2++Ar. The conversion frequency, found to be 376p02 s−1 (p0 being the pressure reduced to 273°K), is compared with the available theoretical values. Owing to the high electron density and large-diameter discharge tube the present method is more suitable for the measurement of this parameter in argon than were previous microwave studies.

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