Photoionization of [Formula Presented] using synchrotron radiation

Absolute measurements of cross sections for photoionization of a statistical admixture of [Formula Presented] in the [Formula Presented] ground state and the [Formula Presented] metastable state are reported in the energy range 40–71 eV at photon energy resolutions ranging from 22 meV to 2 meV. The experiments were performed using synchrotron radiation from an undulator beamline of the Advanced Light Source with a newly developed ion-photon-beam endstation. The data are characterized by multiple Rydberg series of autoionizing resonances superimposed upon a direct photoionization background cross section where some of the observed resonance line shapes show evidence of interference between the direct and indirect photoionization channels. The resonance features are assigned spectroscopically, and their energies and quantum defects are tabulated. The experimental photoionization cross sections are in satisfactory agreement with the predictions from theoretical calculations performed in intermediate coupling using the semirelativistic Breit-Pauli approximation with ten states. The resonances nearest to the ionization thresholds exhibit anomalous behavior with respect to their positions and strengths due to the presence of interloping resonances associated with higher-lying ionic states causing disruption of the regular Rydberg spectral pattern. 5555 2002 The American Physical Society.