Photoionization Cross Section of Helium in the 100- to 250-Å Region

The absolute $K$-photoionization cross section of helium has been measured in the region extending from 100 to 250 Å to a precision of 5% or better. Above 250 Å, where the present results overlap with earlier work performed in this laboratory, there is satisfactory agreement and the data have been combined so as to extend the range of measurements by about 8 Ry above the ionization threshold. It has been possible to join these results with a previous observation made at a higher energy (20.3 Ry) so that helium is the only gas other than Ne for which the cross section is known over a large energy range with comparable precision. This information, combined with the outcome of theoretical calculations and with estimates of discrete oscillator strengths, has led to the evaluation of four sum rules. The calculations attest to the reliability of the observations.