HIGH-RESOLUTION STUDY OF P-1(0) DOUBLE-EXCITATION STATES IN HELIUM

We describe extensive results on the P-1(0) double-excitation resonanc es of He below the N=2-9 ionization thresholds obtained recently by hi gh-resolution photoionization measurements using synchrotron radiation from the SX700/II monochromator at the Berliner Elektronenspeicherrin g fur Synchrotronstrahlung (BESSY). The resonance energies, linewidths , Fano-q parameters, and quantum defects of the various Rydberg resona nces, derived from the experimental data, are compared in detail with the results of ab initio calculations employing the complex-rotation m ethod. All three optically allowed Rydberg series below the N=2 ioniza tion threshold were identified. For 3 less than or equal to N less tha n or equal to-8, the two most intense Rydberg series were resolved bel ow each threshold, and for N=3, one state of a third series could be i dentified. For the higher Rydberg series (N greater than or equal to 5 ), the two neighboring series, N and N+1, generally interfere, which h as a strong influence on the transition intensities, linewidths, and q uantum defects of the principal series. All together, stales of 18 dif ferent Rydberg series in the P-1(0) double-excitation resonance region of He were observed.