VERY NARROW DOUBLY-EXCITED 2(1,0)-N AND 2(-1,0)0N P-1(O) STATES OF HELIUM

We present a detailed theoretical study of the very narrow A = - and A = 0 series seen in the recently reported high-resolution photoionization spectrum of He below the N = 2 threshold [M. Domke et al., Phys. Rev. Lett. 69, 1171 (1992)]. The hyperspherical close-coupling method combined with the multichannel quantum-defect theory is used and the photoionization cross section has been calculated over a wide energy range with high efficiency. Our results are in excellent agreement with the experimental data and compare favorably with recent accurate theoretical works.