Two-photon double ionization of helium by attosecond laser pulses: Evidence of highly correlated electron motion

We apply a recently developed ab initio numerical framework to investigate the angular distributions of the emitted electrons in the immediate proximity of the threshold for the two-photon double ionization of helium. Provided one of the electrons is emitted perpendicular to the laser polarization direction, it is found that the angular distribution of the other electron is characterized by three lobes. The results are similar to those recently reported for the corresponding process in the hydrogen negative ion [R. Nepstad and M. Førre, Phys. Rev. A 84, 021402(R) (2011)], and provide further evidence of highly correlated electron dynamics in the vicinity of the double ionization threshold.