We have calculated the energy and angular distributions for double ionization of He by one photon, over a range of photon energies up to 8 keV. We identify, from structures in the angular distributions, three diR'erent mechanisms for double ionization at high photon energies, and we compare the ratio of cross sections for double to single ionization with experimental data. PACS number(s): 32.80.Fb Double ionization of He by one photon cannot occur without the two electrons interacting with each other, in contrast to single ionization. At high photon energies the dynamics of double ionization continues to be of interest, particularly in view of a recent experiment [1]. We report below results of calculations of the energy and angular distributions for double ionization of He by one photon over a range of energies up to 8 keV. We include electronelectron correlation in both initial and final states, and we identify from the angular distributions three different mechanisms for double ionization of He.One well-known mechanism for double ionization is shakeoffone of the electrons absorbs the photon and the other electron is shaken loose by the sudden change in the effective nuclear charge resulting from the swift departure of the first electron [2]. In collision terms, the shakeoff of the second electron is the consequence of a soft collision with the first one. Shakeoff is effective in producing slow electrons. However, two fast electrons can emerge via the "knockout" mechanismas in shakeoff, one of the electrons absorbs the photon, but as this fast electron exits the atom it undergoes a hard binary collision with the other electron [3 -8]. In both of these mechanisms the large net momentum carried away by the electrons must originate from a hard collision with the nucleus since the photon can impart energy but not momentum to the electrons (we neglect retardation throughout). However, there is a third mechanism whereby almost no momentum need be exchanged with the nucleus; the two electrons, by simultaneously sharing the photon, can leave with nearly equal but opposite momenta, and thereby carry away almost no net momentum [9]. Remarkably, this mechanism is inhibited (but not excluded) for the following reasons: Two identical charged particles, which are otherwise free, cannot absorb radiation since their electric dipole depends only on their centerof-mass coordinate; if there is no external force, the center of mass, and therefore the dipole, does not accelerate, and hence cannot absorb radiation. Consequently, while an isolated electron-positron pair can absorb radiation, two electrons can absorb radiation only in the presence of a third body, in our case the nucleus. Furthermore, due to inversion symmetry, as explained below, the two electrons cannot emerge with exactly equal and opposite momenta.We assume that the light is linearly polarized (along the z axis) and that the atomic states are spin-singlet (we factor out the spin). Unless specified otherwise, we use atomic units. The differential cross section for ...
