THE MULTICHANNEL EIKONAL THEORY OF ELECTRON-HELIUM COLLISIONS .1. EXCITATION OF HE(1(1)S)

Calculations of the integral and differential cross sections for the electronic excitation of the 2(1)L and 3(1)L states of helium are reported. The multichannel eikonal theory employing a 10-channel basis set is used. The lambda and chi-parameters for the 2(1)P and 3(1)P states at 80 eV are also reported. A number of misprints concerning the original multichannel eikonal theory results of Flannery and McCann have occurred in the literature. These misprints are corrected in this paper. A detailed description of both the theoretical and experimental difficulties involved in the determination of the integral cross sections-sigma-1(1)S-->3(1)S and sigma-1(1)S-->3(1)D is provided. In addition, a critique of the assumptions made in previous experimental determinations of the differential cross sections (both composite and magnetic sublevel) for the 3(1)L states of helium is given. A result of this analysis is that the experimental data on the differential cross sections for the 3(1)P state need to be re-interpreted in light of the experimental data of van Linden van den Heuvell et al. A principal conclusion of this paper is that the first Born approximation underestimates experimental data in the intermediate energy region for 1S --> 1D transitions, in contrast to the trend seen in 1S --> 1P transitions. The reason for such a reversal in 1S --> 1D transitions can be understood in terms of the direct (i --> f), and indirect (i --> n --> f, etc) mechanisms which are important in the electronic excitation of the 3(1)L states of He. In particular, in this paper the importance of indirect mechanisms in the excitation of the 3(1)S and 3(1)D is shown, thereby confirming the early results of Chung and Lin and the tentative conclusions of Massey.