Angular distributions in double ionization of helium under XUV sub-femtosecond radiation

We present photoelectron angular distributions resulting from the two-photon direct double ionization of helium, under XUV radiation, by solving the time-dependent (TD) Schrodinger equation. The helium TD wavefunction is expanded in terms of fully correlated multichannel states normalized with incoming-wave boundary conditions. The present study focuses on fields of pulse durations within the subfemtosecond regime and at photon energy of 45 eV where the direct double ionization channel dominates the sequential channel. In addition, at this photon energy, the ejected electrons, resulting from the direct and the sequential path, acquire non-overlapping kinetic energy spectra. Our study reveals a trend for back-to-back ejection asymmetry independently of the kinetic energies of the electrons, thus implying that angular correlations are taking place at the time of the ionization. In addition, for given kinetic-energy sharing, it appears that this asymmetry is developed within an interatomic time interval of subfemtosecond scale, intimately connected with the electron-electron interaction strength.