Resonant Double Photoionization of Li Studied with High Energy Resolution

Employing monochromatized synchrotron radiation of the new VLS-PGM beamline at the Synchrotron Radiation Center (SRC), we have measured with high energy resolution the relative photoionization cross-sections for the formation of Li$^{+}$ and Li$^{2+}$ ions between 148 and 161 eV photon energy. This energy region is characterized by double and triple excitations that lead to strong enhancements in the cross sections, particularly in the Li$^{2+}$ cross section. In an earlier study performed by Huang {\it et al.} \footnote{M.-T.\ Huang, R.\ Wehlitz, Y.\ Azuma, L.\ Pibida, I.A.\ Sellin, J.W.\ Cooper, M.\ Koide, H.\ Ishijima, and T.\ Nagata, Phys.\ Rev.\ A {\bf 59}, 3397 (1999)} only a moderate energy resolution was used. Our high-resolution data exhibit a dramatic resonance structure in the double-to-single ionization ratio not seen before.