The ionisation coefficient in Ar-Hg mixtures

The ionisation coefficient in Ar-Hg mixtures has been measured for E/n values between 6*10 -21 V m 2 and 2*10 -18 V m 2 and for a mercury atom fraction varying from 2*10 -4 to 2*10 -2 . The experiments were performed with a parallel-plate apparatus in which the current is measured as a function of the plate separation at constant electric field. The ionisation coefficient was found to be a function of E/n and the mercury atom fraction only. Small admixtures of mercury lead to a large increase in the ionisation coefficient due to Penning ionisation and associative ionisation. From the experimental results the effective rate coefficient for excitation to the appropriate argon states is obtained as a function of E/n, as well as the ratio of the rate coefficients for depopulation of these excited argon states by collisions with argon and mercury atoms respectively.