Photoionization Cross Sections of Helium

Abstract

The photoionization cross sections of helium have been measured from threshold down to 260 Å. At the spectral head the cross section is found to be 7.3×10⁻¹⁸ cm². The experimental results are compared to available theoretical data. The oscillator strength for transitions into the ionization continuum have been estimated from threshold to 0.01 Å using the present and previously published data. A continuum f value of 1.54 is obtained. Applying the Thornas–Kuhn sum rule one therefore obtains the total oscillator strength for all discrete transitions as f = 0.46.