Triple differential cross sections and nuclear recoil in two-photon double ionization of helium

Triple differential cross sections (TDCSs) for two-photon double ionization of helium are calculated using the method of exterior complex scaling both above and below the threshold for sequential ionization (54.4 eV). It is found that sequential ionization produces characteristic behavior in the TDCSs that identifies that process when it is in competition with nonsequential ionization. Moreover, we see the signature in the TDCSs and nuclear-recoil cross sections of virtual sequential ionization below the threshold for the sequential process.