Recent progress in low energy atomic hydrogen (e,2e) measurements

Abstract
Coplanar (e,2e) cross sections of atomic hydrogen have been measured 1, 2 and 4 eV above threshold. The angular correlations taken with the constant difference angle [MATH] show a slight but significant change of the triply differential cross section if the excess energy is lowered from 2 eV to 1 eV suggesting that the extend of the threshold region in atomic hydrogen is smaller than 2 eV. In the angular correlations taken with [MATH] the two separate peaks found 2 eV above threshold form a single peak if one approaches closer to threshold. The experimental results are compared with the distorted wave Born theory of Jones, Madison and Srivastava (1992) exhibiting deviations with respect to the positions of the peaks and their relative intensities. For fixed angle [MATH] the triply differential cross section 2 eV above threshold exhibits a minimum just at [MATH]. At 1 eV above threshold the minimum has essentially disappeared but the cross section is still not concentrated in an angular region around 90° as could be expected from the Wannier-Peterkop-Rau theory.