GAS DESORPTION FROM AN OXYGEN-FREE HIGH-CONDUCTIVITY COPPER VACUUM CHAMBER BY SYNCHROTRON-RADIATION PHOTONS

An OFHC copper chamber was exposed to 3.75 keV critical energy synchro tron radiation for a total dose of of 2.7 x 10(23) photons/m. After th is dose, photodesorption yields down to 10(-5) molecules/photon were r eached for H-2, CO, and CO2 and 10(-7) mol/photon for CH4, H2O, and O2 and a total of about one monolayer of gas was desorbed. From observat ion of the transient behavior of the desorption at the beginning of an irradiation period after a prolonged interruption, the desorption yie ld was found to be proportional to the coverage and followed an expone ntial decay with increasing photon dose. This is in contrast with the normal behavior where the desorption yield decreases linearly with dos e on a log-log scale. The observations are interpreted by a two-step p rocess involving photons or photelectrons to create a new surface phas e of molecules which subsequently desorb thermally. With die chamber h eated to 70-degrees-C, the photodesorption yields for H2O and O2 incre ased but the others remained relatively independent of the temperature . At the end of the irradiation period a substantial wall pumping was observed in the chamber. For CO, CO2, and O2 it was about 1160, 280, a nd 80 l/s respectively.