Differential electron scattering from positive ions measured by zero-degree ion–atom spectroscopy

Recent developments in quasi-free electron scattering from highly charged ions are reported. Results using a high-efficiency zero-degree Auger projectile spectroscopy setup are presented primarily for collisions of H- and He-like boron beams with H2 targets. A novel technique for the in situ control and determination of the metastable 1s2s3S beam fraction of He-like boron allowed for an accurate determination of absolute double differential scattering cross sections for 180° resonant and non-resonant electron scattering from both ground-(1s2) and mixed-(1s2, 1s2s3S) B3+ states. The observed richly structured B2+ (1s2lnl′) Rydberg series is produced by resonant excitation (RE) from the 1s2 ground state, while the triply excited B2+ (2s2l′2l′′) hollow ionic states are produced by RE from the 1s2s3S metastable state. The consistency of the method was checked by quasi-free electron scattering from H-like B4+ ions giving rise to the 2lnl′ Rydberg series. Differential R-matrix calculations within the electron scattering model for electron–ion scattering from both H-like and ground state He-like boron were found to be in excellent overall agreement. The production of triply excited (2s2l′2l′′) states by RE was also studied for the Z=5–9 isoelectronic sequence. These results convincingly demonstrate that large-angle differential scattering cross section measurements, practically impossible to perform with present day crossed or merged electron–ion beam techniques, are amenable to measurement through high resolution electron spectroscopy of ion–atom collisions, providing some of the most sensitive tests of atomic structure and electron–ion scattering calculations.