Absorption Cross Section of Helium and Argon in the Extreme Ultraviolet

The absorption of radiation of wavelength between 240 A and 1000 A in helium and argon was measured photometrically in a grazing incidence vacuum spectrograph. For helium a sharp absorption edge was observed at 504.3 A. Near this edge the photoionization cross section at 495.7 A was found to be 6.95×{}10^-18 cm². The variation of the cross section as a function of the wavelength was in agreement with the computed curves by Wheeler and Vinti within the experimental error limit. For argon three absorption edges exist at 786.8 A, 778.0 A, and 424.0 A corresponding to the M₃, M₂, and M₁ edges, respectively. Possible autoionization phenomena between the M₂ and M₃ edges were observed. The cross section at the M₂ edge was found to be 3.5×{}10^-17 cm² which is comparable with the value computed by Dalgarno.