A scattered-electron cascaded-photon coincidence experiment on the electron impact excitation, of the 31D state of helium

A modified electron-photon coincidence experiment on the excitation of the 3 1 D state of helium is suggested. The idea is to measure the scattered electrons, which have excited the 3 1 D state, in coincidence with 2 1 P to 1 1 S photons resulting from the 3 1 D to 2 1 P to 1 1 S cascade. Due to their large energy the latter can be detected with higher efficiency than the directly emitted 3 1 D to 2 1 P photons. A theory is developed to show that the proposed experiment yields exactly the same information on the 3 1 D excitation as a conventional coincidence experiment between scattered electrons and 3 1 D to 2 1 P photons. Preliminary experiments have been performed, in which the 3 1 D excitation is measured simultaneously with the 3 1 S and 3 1 P excitation. The coincidence signal due to the 3 1 S excitation, with its isotropic radiation field is used to calibrate the 3 1 P and 3 1 D measurements.