Time-dependent formation of the profile of resonance atomic states and its dependence on the duration of ultrashort pulses from free-electron lasers

We have computed from first principles the dependence of the formation of the resonance profile of the He 2s2p ¹P^o state at 60.1 eV on the duration of the interaction of He 1s^{2 1}S with ultrashort Gaussian electromagnetic pulses for field strengths smaller than 0.04 a.u. The calculations took into account the self-consistent field, the state-specific electron correlations and the mixing of bound with scattering components. Apart from the new physical information that they reveal, such results, and similar ones for other resonance states, can be of use to current developments toward the accurate characterization of ultrashort pulses from free-electron lasers in the regimes of a few femtoseconds to a few hundreds of attoseconds.