(E, 2E) TRIPLE DIFFERENTIAL CROSS-SECTIONS FOR THE SIMULTANEOUS IONIZATION AND EXCITATION OF HELIUM

We present absolute triple differential cross sections (TDCS) measurements for ionization of helium leaving the ion in both n = 1 and n = 2 final states, obtained under asymmetric geometry at an incident energy approximately 5.5 keV and ejected electron energies of 5, 10 and 75 eV. The kinematics are chosen to correspond either to a constant ejection energy, or to a constant energy transfer to the target. Angular distributions are measured at both constant ejection angle (theta(a) mode) and at constant scattering angle (theta(b) mode). In the theta(a) mode experiments, the momentum transfer dependence of the n = 2 triple differential generalized oscillator strength is investigated here for the first time. In both modes, the n = 2 angular distributions show several new features which are not present for the n = 1 ones, and which tend to vanish as the ejected energy is increased. They are attributed to final state interactions between the ejected electron and the excited ion. Comparison with first-order theoretical models shows the inadequacy of a Coulomb wave representation of the ejected electron, while in the R-matrix formalism it is found that a five-state multichannel calculation qualitatively describes the shape (but not the amplitude) of the TDCS measured in the theta(b) mode. Comparison is also made with the photoionization in the dipolar limit where the momentum transfer approaches zero. When integrated over the ejection direction, the double differential generalized oscillator strength ratio for ionization to the n = 1 and n = 2 states is found to agree with a recent first Born close coupling prediction.