Doubly resonant photoionization of He below the N = 2 ionization threshold

We present an extended theoretical study of two-colour doubly resonant photoionization of helium involving two doubly excited states below the N = 2 ionization threshold. We model the effects of a strong laser coupling between the lowest P-1(o) and the selected S-1(e), P-1(e) and D-1(e) doubly excited states on the photoelectron (photoion) yield by taking into account the spatio-temporal structure of the pulses and by considering the effects of the neighbouring resonance states. We show that the laser coupling results in a measurable modification of the line profiles and in a reduction of the photoelectron yield under resonant or near-resonant conditions.