A number of studies of the ionisation behaviour of molecular nitrogen have revealed the importance of N 2(A 3L ;;) metastable particles which are created by direct electron impact excitation or cascade processes. More recent studies using cleaner gas samples have indicated that the ionisation current may be affected by the presence of a second metastable particle. The possible identity of this second metastable particle and the sensitivity of the ionisation current to its presence have been investigated by observing the changes in the pre-breakdown ionisation current as a function of the purity of the N 2 gas samples. Both the spatial and temporal behaviour of the ionisation current were observed. Particular care was required in this study to ensure that stable electrode surface conditions were obtained and the procedures adopted are described in detail. The addition of CO and H2 to the N 2 samples at the parts per million level confirmed the dominant quenching action of CO. Further consideration of the spatial growth of ionisation has demonstrated that an ionisation mechanism involving both N2(A 3L ;;) and N2(a' 1 L;;-) particles provides a plausible explanation for the observations and should be further investigated.
Electron-impact excitation ofXΣg+1(v″=0)to thea″Σg+1,bΠu1,c3Πu1,o3Πu1,b′Σu+1,c4′Σu+1,GΠu3, andFΠu3states of molecular nitrogen