MEASUREMENT OF COHERENCE PARAMETERS FOR THE ELECTRON-IMPACT EXCITATION OF THE 3'D STATE OF HELIUM FROM AN ANGULAR-CORRELATION EXPERIMENT

An angular correlation experiment has been performed to measure three target parameters gamma , Pl and rho 00 for the electron impact excitation of the 31D state of helium. This was done by measuring the angular distribution of the 31D to 21P decay radiation ( lambda =667.8 nm) emitted in the scattering plane in coincidence with the inelastically scattered electrons. The photon intensity perpendicular to the scattering plane was also measured in coincidence with the inelastically scattered electrons. Results are presented over an angular range of 10'-40' at incident electron energies of 60 and 80 eV. The results of gamma and P l are compared with polarisation-correlation experimental data and a variety of theoretical calculations. The measured values of rho 00 are compared with available theoretical data, since no other experimental results for rho 00 are available.