Excitation ionization and double ionization of helium by high-energy photon impact

The ratio of double to single ionization by photoabsorption is calculated for photon impact energies in the range 2–18 keV. Employing the acceleration form of the dipole operator and representing the electron-electron correlation in the final state by a Coulomb distortion factor, R=${\mathrm{\sigma }}^{++}$/${\mathrm{\sigma }}^{+}$ approaches an asymptotic value of 1.66% and reaches a value of about 2.5% at 2 keV photon energy. Comparison with experimental data shows good agreement when contributions due to Compton scattering are taken into account.