Measurement of Absorption Coefficients for Photoionizing Radiations in Low-Pressure Gases with a Space Charge Detector

A Kingdon cage space-charge detector has been used to measure the production of positive ions in a lowpressure gas due to absorption of photons produced by electron impact excitation in another region of the same gas. Absorption coefficients were measured by changing the source to detector separation. High-purity vacuum techniques were used to avoid contamination of the gas. No ionization was found in hydrogen. For nitrogen the molecular cross section was 4.1×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ and for argon the atomic cross section was 3.1×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. Tentative suggestions are made as to the processes responsible for these total absorption cross sections being much larger than the photoionization cross sections for monochromatic radiations in the ionization continuum region.