Absolute Intensity Measurements in the Vacuum Ultraviolet*†

Photo-ionization of the rare gases is proposed as a method to measure the absolute intensities of radiation below 1022 Å, the ionization potential of xenon. The method is based on the assumption that the photo-ionization yield of the rare gases is unity. Experimental evidence is presented to justify this assumption. The relative quantum yield of sodium salicylate is measured and shown to be constant between 400–900 Å. Finally, the photoelectric yield of gold “black” has been measured and found to be about 1% at 1215 Å rising to 4% at 850 Å and remaining constant from there down to 500 Å.