COHERENT EXCITATION OF THE 21P STATE OF HE FOR LARGE MOMENTUM-TRANSFER ELECTRON-SCATTERING

Angular correlations between the scattered electrons resulting from the excitation of the 21P state of helium and the photons emitted in the de-excitation of the state have been measured using a coincidence technique for an incident electron energy of 81.2 eV. The measurements extend over the electron scattering angles 10 degrees to 130 degrees and are the first for large momentum transfer scattering. The angular correlations are analysed to yield the ratio of the differential cross sections for excitation of the degenerate magnetic sublevels of the state and the phase difference between the corresponding excitation amplitudes. The Fano-Macek orientation and alignment parameters are also derived from the data. The measurements provide a more sensitive test of theoretical models than previous small-angle scattering data and it is found that none of the published theoretical values agree with the authors' data.