A Study of the Photodesorption Process for Cryosorbed Layers of $H_{2}, CH_{4}, CO$ or $CO_{2}$ at Various Temperatures between 3 K and 68 K

The vacuum performance of the Large Hadron Collider (LHC) at CERN will depend critically on the photodesorption of gas by synchrotron radiation and the re-adsorption of these molecules back onto the cold surface. The results of photon induced molecular desorption by synchrotron radiation with a critical photon energy of 284 eV for H2, CH4, CO or CO 2 cryosorbed on a stainless steel surface are presented. Most measurements have been carried out in a temperature range from about 3 K to 20 K. Measurements for CO2 were also performed at 68 K. The specific method used for this study has been to pre-deposit a known quantity of gas onto a cold surface, to irradiate the surface with a known photon dose and to measure the quantity of gas remaining on the cold surface by recording the pressure during warm-up. The average photodesorption yields of all gas species were found to increase with increasing surface coverage and to reach a saturation value. For H2 this value is approximately 0.5 for a coverage exceeding 10**17 molecules/cm**2 , for CH4 and CO2 saturation occurs at about 0.5 molecules/photon at ~10** 19 molecules/cm**2 , while the corresponding final value for CO is about 0.04 molecules/photon at this coverage.