Linear and circular dichroism in the double photoionization of helium

K. Soejima$^\dagger$, A. Danjo$^\dagger$, K. Okuno$^\ddagger$,
A. Yagishita$^\sharp$, A.S. Kheifets$^\star$ and I. Bray$^\ast$

$^\dagger$Graduate School of Science and Technology,
Niigata University, Niigata-shi, Niigata 950-21, Japan

$^\ddagger$Department of Physics, Tokyo Metropolitan University,
Hachioji-shi, Tokyo 192-03, Japan

$^\sharp$Photon Factory, National Laboratory for High Energy Physics,
Tsukuba-shi, Ibaraki 305, Japan

$^\star$ RSPhysSE, Australian National University, Canberra
A.C.T. 0200,
Australia.

$^\sharp$ESM Centre, Flinders University
of South Australia, GPO Box 2100, Adelaide S.A. 5001, Australia

Linear and circular dicrhoism in the helium double photoinization has
been studied, both experimentally and theoretically, at equal and
unequal energy sharings at the excess energy of the electron pair of
9eV above the threshold. In contrast to the previous work of Mergel
{\em et al} [Phys. Rev. Lett.  {\bf 80}, 5301 (1998)] good agreement
has been found between theory and experiment in all cases