Helium has a higher 1st ionization energy (24.58eV) than N(adsbygoogle = window.adsbygoogle || []).push({}); _{2}(15.6eV) and O_{2}(12.06eV). For an atmospheric room-temperature helium, why it is easier to get ionized than the daily life air under a same discharge setup? For example, for the Paschen curves, N_{2}locates at the left of He which means that for the same voltage and pressure, N_{2}requires us to move electrodes closer.

The most common answer I heard of this is that N_{2}is diatomic molecule which has more degrees of freedom than helium has. However, I don't think this can explain the following fact that N_{2}has a higher rate coefficient of electron impact ionization than He has, based on the calculation of the Boltzmann solver "BOLSIG+", even for a mean electron temperature (Te) much lower than the ionization thresholds of N_{2}. Considering the electron temperature distribution function (EEDF) is near Maxwellian, if the mean Te is very low, the rate of N_{2}should higher.

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# Why helium is easier to ionize than N2?

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