Measurement of the ionisation coefficients in nitrogen and methane mixtures

The Townsend first ionisation coefficient alpha /p 20 and the total secondary coefficient gamma T have been measured for nitrogen and methane mixtures by the steady-state Townsend method for 40<or approximately=E/p 20 <or approximately=500 V cm -1 Torr -1 (121<or approximately=E/N<or approximately=1520 Td). The results show that the alpha /p 20 of the mixtures lies between the alpha /p 20 values of the respective pure gases and that the alpha /p 20 of the mixture may be represented by a linear function of the fractional methane partial pressure k for E/p 20 <or approximately=100 V cm -1 Torr -1 , but a linear relationship tends to break down at higher E/p 20 . The results also show that the gamma T of the 90% nitrogen and 10% methane mixture increases with E/p 20 and that the gamma T decreases rapidly with k at a fixed E/p 20 . A possible explanation of this rapid decrease in terms of quenching of the excited states of the nitrogen molecule by the methane molecule is given.