Precise Double-Photoionization Data for Na and K

We have measured precise double-to-single photoionization ratios and double-photoionization cross-sections of sodium and potassium near threshold. A previously discovered scaling law \footnote{R.\ Wehlitz and S.\ B.\ Whitfield, J.\ Phys.\ B {\bf 34}, L719 (2001).} allows us to conveniently compare the energy dependence of the double-to-single photoionization ratio by scaling the energy axis. Recently, we have also found a scaling law that enables us to predict the absolute double-to-single photoionization ratio \footnote{J.B.\ Bluett, D.\ Luki\'c, S.B.\ Whitfield, and R.\ Wehlitz, Nucl. Instrum. Methods B, {\bf 241}, 114 (2005).}. We have applied this scaling law to our new data and found excellent agreement. Previous tests of this scaling law were limited to systems where electrons were emitted from $s$-shells. However, in the cases of Na and K a $p$ electrons is participating in the double-ionization process. Interestingly and in spite of the different orbital, the scaling law is still valid.