Ionization dynamics in double resonance involving autoionizing states in helium: the effect of pulse shapes

We discuss the dynamics of doubly resonant ionization in helium involving the strong coupling of two doubly excited autoionizing resonances, under the influence of XUV radiation of short duration (DESY FEL) and a synchronized optical laser. We explore the possibility of using the photoelectron spectra to extract information about the pulse shape of the FEL. We present in addition new calculations of the expected AC Stark splitting due to the strong coupling of the two resonances and show that, within the range of intensities and pulse durations expected in the experiments, additional structure, beyond the usual doublet, is to be expected.