Electron-photon angular correlations from the electron impact excitation of the 2s and 2p electronic configurations of atomic hydrogen

The angular correlation of Lyman-alpha photons detected in coincidence with the 10.2 eV energy-loss electrons have been measured for 54.4 eV incident electrons exciting the n=2 levels of atomic hydrogen. For electron scattering angles in the range from 10 to 140 degrees the in-plane angular correlations were measured for photon angles from 10 to 140 degrees . These data were interpreted in terms of lambda , the ratio of the differential cross sections mod a ml mod 2 for magnetic sublevel m l excitation, and R, the real part of the product (a 0 a 1 ) in which the angle brackets indicate summation over the spin states. The error bars on these values are significantly smaller than on previous measurements of Williams (1975), Hood et al (1979) and Weigold et al. (1980). No existing theory predicts values in agreement with the present data. An angular correlation in which the photon detector axis is perpendicular to the scattering plane is shown to measure directly the 2p state angular differential cross section as well as the ratio of that cross section to the total 2p cross section. Also, by using an appropriate quenching pulse, the 2s state angular differential cross section, sigma (2s), is measured.