Measurements of the ultrasoft X-ray absorption of Ar, Ne, N2, O2, CH4, He and H2

New measurements are presented of the X-ray absorption cross sections of argon, neon, oxygen, nitrogen, methane, carbon, helium and hydrogen in the wave-length interval 23·7 to 82·1 Å. The values obtained are compared with the theories of Hall and Bearden and in the case of helium those of Huang and Stewart and Webb. Further support is provided for Bearden's K-shell theory, but the agreement for the L-shell absorption region is shown to be poor. The empirical predictions of Henke et al. are also not supported. At shorter wavelengths the helium results lie closer to the theory of Stewart and Webb, based on calculation of the acceleration matrix element, than to Huang's predictions based on the momentum matrix element. An empirical absorption formula relating cross section, wavelength and atomic number is given capable of predicting the experimental results satisfactorily.