Double-photoionization studies of Ne, O, and N from threshold to 280 eV

The photoionization cross sections for double ionization of Ne by single photons have been measured from threshold to 280 eV. The results presented are in considerable variance with other published data. These cross sections are compared with those for ${\mathrm{N}}^{2+}$ and ${\mathrm{O}}^{2+}$. It is found that the ratio of the oscillator strength for double ionization to the total photoionization oscillator strength for each of these atoms is proportional to N-1, where N is the total number of electrons in the atom.