Photoionization Cross Section of Neon in the 80 to 600 Å Region

Abstract

The paper presents measurements of the absolute photoionization cross section of neon in the soft x-ray region (80-600 Å) to an accuracy of 5% or better. Increased reliability of measurements has been brought about by various technical improvements such as the use of an absorber cell with suitable windows and the use of a Geiger-Mueller tube specially designed for counting photons whose energy was in excess of 40 eV. At the $L_{2,3}$ threshold (575 Å) the cross section $\sigma$ has a value of 6.3 Mb, rises to a peak value of 10.0 Mb at 375 Å, and then decreases to 7.8 Mb at the $L_1$ edge (about 261 Å). The range of measurements has been extended to 10 Ry above the ionization threshold. Consequently, it has been possible to join the results with previous observations made at high energies. With the exception of He, neon is the only other gas for which the value of $\sigma$ is known over a large energy range with sufficient precision. This information, combined with estimates of discrete oscillator strengths, has led to the first successful evaluation of the Thomas-Reiche-Kuhn sum rule.