Relative measurements of the Balmer α 3S, 3P, and 3D excitation cross sections produced by electron impact on atomic hydrogen

The first measurement of the relative cross sections for excitation of the 3S, 3P, and 3D states of atomic hydrogen, produced by electron impact is presented. The incident electron impact energy ranged from near threshold (12.2eV) up to 500 eV, when the results for the optically allowed 3P cross section are normalized to the Born prediction, the 3S and 3D cross sections as derived from the measured values also fall within the Born values at this energy. As the impact energy is lowered, the 3S and 3P excitation cross sections fall below the Born predictions, as do the 2S and 2P excitation cross sections. However, the 3D excitation cross section rises above the Born prediction by about a factor of two for impact energies below 50 eV. An argument is presented that the validity of the Born prediction for a ΔL = 2 transition is questionable. The cross sections are compared with other existing theories as well as the Born predictions. No satisfactory agreement is found for the 3D case. The total H/sub alpha/ cross section is presented over the same energy range and normalized as above. A comparison is again made with the existing theories and the experiment of Kleinpoppen (1968). The present results rise considerably above those of Kleinpoppen at energies below 50 eV, and fall between the Glauber and Born predictions at lower impact energies. Since Stark mixing of the n = 3 sublevels could mix these populations before radiation, and thus produce erroneous cross section determinations. A known electric field was therefore placed in the interaction region and the H/sub alpha/ signal profile studied. An unexpected asymmetry of this H/sub alpha/ signal profile was observed upon reversal of the electric field, and this was interpreted as a coherent excitation of pairs of Stark mixed (nLJm/sub J/) and (nL +- 1Jm/sub J) substates, an effect never before observed in crossed beam measurements. (Diss. Abstr. Int., B)