Electron transport and ionization in CHF3–Ar and CHF3–N2 gas mixtures

The pulsed Townsend technique has been used to measure the electron drift velocity, the density-normalized effective ionization coefficient (? ? ?)/N (? and ? are the ionization and attachment coefficients, respectively), the density-normalized longitudinal diffusion coefficient NDL, and the ratio between the longitudinal diffusion coefficient and the electron mobility DL/K, in CHF3 mixed with Ar and N2, over a wide range of the density-normalized electric field strength E/N, from 0.2 to 400?Td (1 Td = 10?17?V?cm2). The CHF3 content in the mixtures was varied between 1% and 50%. Regions of negative differential conductivity (NDC) appear in the plots of the electron drift velocity as a function of E/N. This effect is more pronounced for the CHF3?Ar mixtures than for the CHF3?N2 ones, since it results from the presence of a Ramsauer?Townsend minimum in the momentum transfer cross section for Ar, which is absent in N2. For the CHF3?N2 case, a shallow region of NDC is observed, and it is thought to be due to various inelastic collision processes between the electrons and the buffer gas, and also to the steep fall of the scattering cross sections for CHF3 at low electron energies. Additionally, the dependence of NDL on E/N displays well-defined minima at low E/N, which are a result of the strong inelastic energy loss of the electrons. The effective ionization coefficients were found to be weakly dependent on the concentration mixture for CHF3?N2, while a strong dependence on this parameter was found for the CHF3?Ar mixture. In all cases, the attaching character of the mixtures was found to be very small. For low E/N (< 20%) in the CHF3?N2 mixture it was observed that the values of (? ? ?)/N are higher than those expected due to electron impact. We believe that Penning ionization may be responsible for this effect.