Ionization in positive ion sheaths

It was found that the positive ion current to a plane auxiliary collector placed in a neon discharge had about twice the value expected from the equations of Langmuir and Mott-Smith. This increase must be due to an ionization within the sheath surrounding the electrode or to an emission of electrons from the surface of the electrode. Four different possible causes of the increase are analyzed and relations between the voltage drop V, total current to the collector i, and sheath thickness x are obtained. Comparison with data shows that the increase in i is probably caused by the ionization of the metastable atoms within the sheath by radiation from the discharge. The relations; V = A(Bx^(4/3) + Cx^(8/3)), and i = (i0 + I0x^2)/2 hold, and check fairly well with the three experimental curves. Considerations of atomic energy states of the metastable atoms show that this ionization would be most marked in the noble gases, and almost nonexistant in mercury vapor, which was the gas investigated by Langmuir and Mott-Smith