Vv. Anashin et al., A study of the photodesorption process for cryosorbed layers of H-2, CH4, CO or CO2 at various temperatures between 3 and 68 K, VACUUM, 53(1-2), 1999, pp. 269-272
The vacuum performance of the large hadron collider (LHC) at CERN will depe
nd critically on the photodesorption of gas by synchrotron radiation and th
e re-adsorption of these molecules back onto the cold surface. The results
of photon-induced molecular desorption by synchrotron radiation with a crit
ical photon energy of 284 eV for H-2, CH4, CO or CO2 cryosorbed on a stainl
ess steel surface are presented. Most measurements have been carried out in
a temperature range from about 3 to 20 K. Measurements for CO2 were also p
erformed at 68 K. The specific method used for this study has been to pre-d
eposit a known quantity of gas onto a cold surface, to irradiate the surfac
e 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 phot
odesorption yields of all gas species were found to increase with increasin
g surface coverage and to reach a saturation value. For Hz this value is ap
proximately 0.5 for a coverage exceeding 10(17) molecules/cm(2), for CH4 an
d CO2 saturation occurs at about 0.5 molecules/photon at similar to 10(19)
molecules/cm(2), while the corresponding final value for CO is about 0.04 m
olecules/photon at this coverage. (C) 1999 Elsevier Science Ltd. All rights
reserved.