Angular Correlation Parameters for Excitation of the 21P State of Helium and the 2p State of Atomic Hydrogen

Abstract

Angular correlations between the scattered electrons resulting from the excitation of the 21P state of helium and the photon emitted in the deexcitation of the state have been measured using a coincidence technique for an incident electron energy of 81.2 eV. Similar correlations have been measured for the excitation of the 2p state of atomic hydrogen by 100-eV electrons. These measurements are the first for large-momentum-transfer scattering. The angular correlations are analyzed to yield the ratio of the differential cross sections for excitation of the degenerate magnetic sublevels of the state and, in helium, 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. None of the published theoretical values agrees with our data.