Multipole characterization of the electron loss to the continuum cross section at asymptotically high velocities

A powerful new technique, using a two-dimensional position sensitive multichannel plate detector coupled with a spherical sector electron analyzer, allows rapid data collection of doubly- and triply-differential electron spectra. This device is used to collect spectra, doubly differential in electron energy and emission angle, for the electron loss to the continuum cross section, where an electron originally bound to the incoming projectile is excited just over the ionization limit into the continuum of the ion, for O⁵⁺ projectiles incident on helium and argon gas targets at energies of , 82, and 105 MeV at the Holifield Heavy Ion Research Facility at Oak Ridge National Laboratory. By making a multipole expansion of the cross-section in the projectile frame, and appropriately including the effects of the electron analyzer, a general characterization of the electron loss to the continuum cusp (ELC) shape is made. These results are directly compared with the recent theory of J. Burgdörfer, which derives the multipole content for the ELC cross-section, and excellent agreement is found for the case of argon targets at the highest two energies, where the theory is expected to be more reliable. The lack of good agreement with helium target data suggests the need for better treatment of electron-electron interactions within the theoretical framework, which are expected to be proportionately more significant for helium targets. Also, the importance of this approach in obtaining higher multipole information and some possible future investigations are discussed.