An apparatus for the study of photochemistry in the extreme vacuumhyphen;ultraviolet is described. The light source and photolysis vessel were separated by a fasthyphen;flow separation system, so that no windows were needed. Using a microwave discharge in helium as the light source, photohyphen;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 ionhyphen;chamber. Ion currents up to 25 mgr;A were obtained in methane, corresponding to a useful incidenthyphen;light intensity of about 3times;1014photons/sec. This light is thought to be mostly the helium resonance line at 584 Aring;, 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.
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