Double Photoionization of Magnesium from Threshold to 70 eV

We have determined the double-to-single photoionization ratio of magnesium from 22 to 70 eV photon energy using monochromatized synchrotron radiation and the ion time-of-flight method\footnote{R.\ Wehlitz, P.N.\ Jurani\'c, and D.V. Luki\'c, PRA {\bf 78}, 033428 (2008)}. Our results compare reasonably well with recent theoretical calculations\footnote{A.S. Kheifets and I. Bray, Phys. Rev. A {\bf 75}, 042703 (2007)} in the non-resonant energy region below 54 eV. Here, the photon energy dependence of the ratio can be modeled by a suitably scaled helium double-to-single photoionization ratio. However, to our surprise, a previously proposed scaling model for that ratio does not work in the case of Mg. From the near-threshold double-photoionization cross section we estimate the range of validity of the Wannier threshold law to be about 0.7 eV with a rather small cross section near threshold. At photon energies above 54 eV the ratio is strongly affected by $2p \rightarrow n\ell$ inner-shell excitations and rises sharply from a non-resonant value of about 1\% below the $2p$ thresholds to about 1000\% above thresholds\footnote{R.\ Wehlitz and P.N.\ Jurani\'c, Phys.\ Rev.A {\bf 79}, 013410 (2009)}.