Interaction between overlapping doubly excited resonance series for the He atom

We present a detailed theoretical study on the photoionization from the bound excited 1P state of He to continua dominated by multiple doubly excited resonance series embedded in multiple single-ionization channels between the N = 2 and N = 3 thresholds. In addition to identifying the individual resonance series, our investigation has also demonstrated unambiguously the level crossing between overlapping resonance series and how the closely spaced pairs of resonances migrate as energy increases.