The photoionization cross section of Mgr near threshold in the region between 160 -100 nm was studied using synchrotron radiation, an atomic-beam technique, and a time-of-flight mass spectrorneter for the selective detection of the Mg+ photoions. Special attention was given to the signals of the double electron resonances 3pns and 3pnd 'Pl in comparison with theoretical calculations and recent experiments.

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Ca I atoms were excited by pumping the Ca I 3p64s2 1S0 → Ca I 3p64s4p 1P1 transition using a cw ring-dyelaser. In a second step the atoms were excited by synchrotron radiation to core resonances 3p53dnln' l' which mainly decay by autoionization. Varying the photon energy of the synchrotron radiation the partial cross-section 3p64s4p 1p1 → 3p64sεl were studied

Pumping the resonance transition Ca 4s2 1S0 to 4s4p 1P1 at lambda =422.7 nm with the linear polarized light of a ring dye laser the subsequent excitation by the linear polarized VUV light of the synchrotron radiation source BESSY was used to investigate the photoionization process from laser-excited aligned Ca atoms. Using the dependence of the photoelectron signals on the polarization axes of both radiation sources the symmetry character of core-resonances 3p53d4s4p was determined.