PHOTON-INDUCED MOLECULAR DESORPTION FROM CONDENSED GASES

With a view to predicting the behaviour of the cold vacuum system of t he CERN Large Hadron Collider (LHC), measurements are presented of the synchrotron radiation induced desorption from thick layers of H-2, CH 4, CO and CO2 condensed on stainless steel at 77 K to 68 K for CO2, 4. 2 K for CO and CH4 and to 3.3 K for H-2. The critical energy of the ra diation was 284 eV and up to 10 monolayers of CO2 or 20 monolayers of CO, CH4 and H-2 were condensed. Photodesorption yields from condensed H-2 at 3.3 K were found to be about five molecules per photon at a few monolayers. The photodesorption yields from approximately the same th icknesses of condensed CH4 and CO at 4.2 K and CO2 at 68 K were found to be at least three orders of magnitude lower than that for H-2 and a pparently, within the limits of the experiment, independent of the sur face coverage. Sticking coefficients for CO2 were measured at 77 K and found to increase almost linearly with coverage, reaching a value of 0.2 at approximately 20 monolayers. An adsorption isotherm for CO2 on stainless steel at 77K was also measured. (C) 1997 Elsevier Science Lt d. All rights reserved.