Crossed-Beam Measurement of the Electron-Impact Polarization of Helium 4922 Å Optical Radiation

Nonzero threshold polarization of helium 4922 {} optical radiation, as first reported by the author, has since been observed by Heddle and Keesing. A recent experiment designed to minimize elastic gas scattering of the electrons has involved He pressures ranging from 10^-7 to 10^-4 Torr, background pressures of 3×{}10^-8 Torr, an optical resolution of less than 10 {} half-intensity width, zero-magnetic-field focusing of electrons, and gas scattering of the electron beam of less than 1%. The energy resolution of the electron beam was of the order of 0.2-eV half-intensity width. Customary photon-counting rates varied from background to 100 counts/sec with a background of about 1 count/sec. Thus, obtaining satisfactory counting statistics presented no appreciable problem, except at threshold. The results compare with those reported earlier, suggesting that the energy dependence of the polarization, which dips to a near-threshold minimum of approximately 30%, is a real effect. The observation has not been adequately explained.