Photochemistry in the Photo-Ionization Region. I. Apparatus and Techniques

An apparatus for the study of photochemistry in the extreme vacuum-ultraviolet is described. The light source and photolysis vessel were separated by a fast-flow separation system, so that no windows were needed. Using a microwave discharge in helium as the light source, photo-ionization in methane and the rare gases has been studied. By using a filter to attenuate the light intensity, it was possible to measure saturation ion currents in the spherical photolysis vessel, which also served as an ion-chamber. Ion currents up to 25 μA were obtained in methane, corresponding to a useful incident-light intensity of about 3×1014 photons/sec. This light is thought to be mostly the helium resonance line at 584 Å, strongly reversed. A ``background'' ion current was observed with only helium in the photolysis vessel, and has been tentatively attributed to electron emission from the collecting electrode induced by collision of metastable helium atoms.