Electron beam excitation studies of helium
Dennis, Wiley Sam
Walters, G. King
Master of Arts
A comparison is made of relative number densities of excited states produced in Helium gas excited by an RF discharge and by the high energy (200kev, maximum) electron beam, it being found that the 33D states are relatively much more numerous in the discharge; a likely explanation is that free electrons in the discharge are energetic enough (~35ev), due to energy supplied by the RF field, to excite 3 JD states directly, while beam electrons and secondary electrons which they eject have, respectively, too much and too little energy to produce such states. Since triplet states cannot be produced directly by the high energy beam, the mechanism of their production is investigated. Electron-ion recombination is eliminated due to lack of a recombination spectrum and due to an observed linear dependence of triplet line intensities on beam current. Excitation by ejected secondary electrons is eliminated on the basis of a calculated cross section for the process of 10-28 cm2 or less. Excitation Transfer from singlet states is concluded to produce the triplets; pressure dependence of the 5016A (3 P-2 S) line is found to follow closely the predictions of the Two Level Transfer Model. A modification of this model which takes account of the fact that triplet states are not produced directly by the beam allows the prediction of the pressure dependence of the 5876A (3 D—2 P) line. Certain technical questions are investigated. It is found that light output of the chamber is more than ample (160kev, 0.02 m.a.) and that x-ray induced noise in the EMI photomultiplier is not excessive. The 0.000125 inch Havar foils used are found to carry 0.3 m.a. at 160kev. The beam is found to be scattered by the foil and then by the A1 back plate of the chamber, causing electrons to strike the walls of the chamber and the glass to fluoresce and become brown tinted with use. A modified chamber design to rectify this situation is suggested. The possible uses of the technique of high energy electron beam excitation of gases and liquids are discussed. Mobility studies of ions and excited species in gases and liquids should be possible. The fact that the high energy beam excites only those states with the same spin multiplicity as the ground state (singlets in Helium) should facilitate the measurement of excitation transfer cross sections between states of different multiplicity. Finally, the process of optical pumping of Helium and the uses of beam excitation therein are dismissed. The relaxing effect of the beam’s magnetic field gradient upon the optically polarized He3 nuclei is investigated, and it is concluded that the application of a small external field will be necessary but that such a field should not affect the beam optics substantially.