Detailed observations of photo-accessible triplet doubly excited states in helium

High resolution synchrotron radiation has been used to make extensive measurements of the doubly excited states converging on the He+(N = 2) ionization threshold of helium. The excitation of two series of triplet doubly excited states, 2pnd(3Do) and sp2,n−(3Po), have recently been observed by Penent et al (2001 Phys. Rev. Lett. 86 2758). In this paper the excitation of a third weakly excited series of triplet doubly excited states, 2pnd(3Po), which was suggested by the previous measurements, is confirmed. The metastable atom yields, resulting from the radiative decay of these three observable series of triplet doubly excited series, are reported and compared to the recent theoretical calculations of Žitnik et al (2002 Phys. Rev. A 65 032520).