Electron-impact coherence parameters for excitation of the 1P states in helium.

Abstract

First-order many-body theory (FOMBT) has been used to calculate differential cross sections (DCS's) and electron-impact coherence parameters (EICP's) associated with excitation of the ${n}^{1}$P (n=2,3) electronic states of helium for incident-electron energies in the range 22 eV \ensuremath{\le}E\ensuremath{\le}500 eV. The DCS's predicted by FOMBT agree reasonably well with the available experimental data and in the 50 eV \ensuremath{\le}E\ensuremath{\le}500 eV energy range, as well as, or better than, those predicted by any other theory. The EICP's predicted by R-matrix methods are found to agree better with the available experimental data in the threshold region (E\ensuremath{\le}30 eV) than those predicted by FOMBT. However, in the intermediate-energy region, 50 eV \ensuremath{\le}E\ensuremath{\le}500 eV, FOMBT provides EICP results that are in semiquantitative agreement with the available data. Partial-wave decompositions of the 〈${L}_{\ensuremath{\perp}}$〉 parameter at various incident-electron energies show that the structure observed in this parameter is caused by interference among the free-electron partial waves and that the small-angle characteristics of this parameter are determined principally by low-l partial waves, i.e., by incident electrons with small-impact parameters. Additional experimental measurements and results from more elaborate calculations at intermediate and large scattering angles and for intermediate and high incident-electron energies are needed to resolve a number of issues associated with the interpretation of the EICP's.