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 ʃ value of 1.54 is obtained. Applying the Thomas-Kuhn sum rule one therefore obtains the total oscillator strength for all discrete transitions as ʃ= 0.46.
JAMES A. R. SAMSON, "Photoionization Cross Sections of Helium," J. Opt. Soc. Am. 54, 876-878 (1964)