energy quantum is used for the ionization of just one gas molecule. Firstly it follows that the ionization energy (that is, the theoretically necessary energy to ionize) of a molecule cannot be larger than the energy of an absorbed light energy quantum. Taking J as the (theoretical) ionization energy per gram equivalent, we have:
According to Lenard's measurements for air the largest wavelength that has an effect is about 1.9·10-5 cm, so
An upper limit for the ionization energy can also be obtained from the ionization voltage in rarefied gases. According to Stark  the smallest measured ionization voltage (for platinum anodes) is for air about 10 volt.  We have thus for J an upper limit 9.6·1012, which is nearly the same as the one just found. There is another consequence that in my mind is very important to verify. If every light energy quantum ionizes one molecule then the following relation must exist between the absorbed quantity of light L and the number j of thereby ionized gram molecules:
If our understanding reflects reality this relation must hold for every gas that (at the particular frequency) has no absorption that isn't accompanied by ionization.
Bern, march 17, 1905
- J. Stark, Die Elektricität in Gasen p. 57. Leipzig 1902.
- within the gas the ionization voltage for negative ions is nonetheless five times larger