Scattered electron-photon angular correlations for the 21P state of helium below the n=3 threshold

Angular correlations between scattered electrons which have excited the 2 1 P state of helium and the 58.4 nm photons emitted in its subsequent decay have been measured at an incident electron energy of 22.0 eV and for scattering angles up to 120 degrees . The data reported, at 800 meV above the 2 1 P excitation threshold, are considerably closer to the threshold energy than all previous angular correlation studies for any electron induced excitation or ionisation process. The higher values of the parameter lambda derived from the angular correlations relative to those previously measured at 24.0 eV are consistent with simple threshold predictions.