New interseries interferences in doubly-excited helium

We present spectra of doubly-excited P-1(o) states in helium below the N = 4, 6, 8, and 9 ionization thresholds measured with a resolution of Delta E congruent to 2.0 meV. A number of hitherto unobserved interseries interfere nces are reported. The spectra below the N = 8 and 9 ionization thresholds are assigned and analyzed using theoretical results obtained with the compl ex rotation method and diagonalization of sparse matrices. The resonances b elow the N = 9 ionization threshold reveal unexpected high values and stron g fluctuations of the Fano parameter q. (C) 1999 Elsevier Science B.V. All rights reserved.