Very narrow doubly excited 2(1,0)−nand 2(-1,0)0n1Postates of helium

Abstract
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. 66, 1306 (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.