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Polarization of electron impact light from helium Whitteker, James Howard

Abstract

The polarization of light from helium atoms excited by the impact of low energy electrons has been measured for the spectral lines 2³P - 2³S (10,829 Å) and 3³P - 2³S (3889 Å). An electron beam carrying a current of 10μA was directed into helium gas at a pressure of 4 x 10⁻³ torr or less. Polarization was measured as a function of electron energy in a range from the excitation threshold (approximately 23 electron volts) to 50 e.v. For the 2³T - 2³S line, this work represents the first reported measurement of this type. There is special interest in the value of polarization near the excitation threshold. The theoretical threshold polarization for both lines studied in this thesis is 36.6%. In the experiment of this thesis, the observed polarization of the 2³P line rises to 21% near threshold, and by means of a curve fitting procedure may be extrapolated to 32 ± 6%. The polarization of the 3³P - 2³S line rises to 11% and may be extrapolated to 15 ± 3%.

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