Proportionality of electron-impact ionization to double photoionization

A comparison is made between the cross section ${\mathrm{\sigma }}_{\mathit{e}}^{+}$ for electron-impact ionization of singly charged ions and the ratio ${\mathrm{\sigma }}^{2+}$/${\mathrm{\sigma }}_{\mathit{t}}$(abs) of double photoionization to the total photoionization cross section for N, O, and Ne. The photon- and electron-impact data were found to be proportional to each other to within about 9% for the first 70 eV above threshold. A study of the literature data shows that this proportionality between photon- and electron-impact data is a general phenomenon. The limits of the proportionality are unknown.