Absorption cross sections of argon and methane have been determined in their respective ionization continua, with particular emphasis on the short-wavelength range from 600 to 170 Å. A line emission spectrum from a high-voltage condensed spark discharge in nitrogen was used as a light source, and a maximum value of the cross section for argon beyond the 2P½0 edge was measured to be 36.0× 10-18 cm2. The oscillator strength for the transition into the continuum has been obtained by integrating the experimental curve of cross sections versus wavenumber from the ionization limit of argon down to about 170 Å and was found to be ƒ= 5.5. Similarly, short-wavelength cross sections in methane were combined with earlier results at longer wavelengths, and a total ƒf 7.6 was obtained. A comparison with the theoretical ƒ values indicates the need of more refined calculations for the photo-ionization of argon and methane.
OM PRAKASH RUSTGI, "Absorption Cross Sections of Argon and Methane between 600 and 170 Å," J. Opt. Soc. Am. 54, 464-465 (1964)