  We investigate the photoionization spectrum of helium by attosecond XUV
pulses both in the spectral region of doubly excited resonances as well as
above the double ionization threshold. In order to probe for convergence, we
compare three techniques to extract photoelectron spectra from the wavepacket
resulting from the integration of the time-dependent Schroedinger equation in
a finite-element discrete variable representation basis. These techniques are:
projection on products of hydrogenic bound and continuum states, projection
onto multi-channel scattering states computed in a B-spline close-coupling
basis, and a technique based on exterior complex scaling (ECS) implemented in
the same basis used for the time propagation. These methods allow to monitor
the population of continuum states in wavepackets created with ultrashort
pulses in different regimes. Applications include photo cross sections and
anisotropy parameters in the spectral region of doubly excited resonances,
time-resolved photoexcitation of autoionizing resonances in an attosecond
pump-probe setting, and the energy and angular distribution of correlated
wavepackets for two-photon double ionization.
