SINGLE AND DOUBLE PHOTOIONIZATION CROSS-SECTIONS OF THE HE- 1S2S2P P-4(0) STATE AT AND FAR FROM THRESHOLD, FROM THE MANY-ELECTRON, MANY-PHOTON THEORY

We report values for the single-photoionization cross sections to the S-3 and P-3(0) channels and for the total double photoionization cross section, sigma(++)(E), of the metastable He- 1s2s2p P-4(0) state, obt ained from the implementation of the state-specific, complex eigenvalu e many-electron, many-photon theory (MEMPT). The sigma(++)(E) rises fr om 0 at E = 0 to a maximum of about 1.0 x 10(-20) cm(2) at E similar t o 3-5 eV and dies out around 30 eV. For this quantity, comparison with the measurements of Bae et al for E = 0.0-0.4 eV and with the semicla ssical calculations of Crothers and Lennon, reveals disagreement. The effect of initial-state correlation in this case is quantitative but n ot qualitative.