Double Excitation of Helium by Electron Impact
Abstract
This study is devoted to the theory and calculation of cross sections for the electronimpact excitation of doublyexcited states in helium which are stable to autoionization. The cross sections are found to exhibit sharp peaks just above the threshold energies for excitation and to decrease rapidly with further increase of energy. The maximum value of the BornOppenheimer cross section for excitation of the (2p)^{2} ^{3}P_{g} state is about 6×10^{4} (a_{o})^{2} and occurs at approximately 11 eV above threshold. The cross sections for excitation of the 2p3p ^{1}P_{g}, 2p3d ^{3}D_{u}, and 2p3d^{1}D_{u} states are all less than 35 b. The cross section for the (2p)^{2} ^{3}P_{g} state has been calculated according to the BornOppenheimer, distortedwave, and twostate strongcoupling approximations. In all of these cases the maximum amplitude of the scattered beam occurs at right angles to the direction of the incident electrons. The cross sections of the potentialandexchangedistortion method (DEW) and of the complete twostate strongcoupling approximation are virtually identical. The BornOppenheimer approximation produces remarkably similar results. Rigorous upper bounds to the energies of the 2p3p ^{1}P_{g}, 2p3d ^{3}D_{u}, and 2p3d ^{1}D_{u} states of helium are reported. Finally, the cross sections are calculated in BornOppenheimer approximation for electronimpact excitation of the (1s)^{2}(2p)^{2} ^{3}P_{g} and (1s)^{2}2p3p ^{3}P_{g} states of beryllium. The peak values of these cross sections are 12 (a_{o})^{2} and 2(a_{o})^{2}, respectively.
 Publication:

Physical Review
 Pub Date:
 October 1964
 DOI:
 10.1103/PhysRev.136.A73
 Bibcode:
 1964PhRv..136...73B