Regensburg 1998 – wissenschaftliches Programm

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VA: Vakuumphysik und Vakuumtechnik

VA 3: Große Vakuumsysteme

VA 3.3: Fachvortrag

Dienstag, 24. März 1998, 10:10–10:30, H48

Normal Incidence Photodesorption Yield of Physisorbed Molecules at 4.2 K on a Stainless Steel Surface — •V. Baglin — CERN, Geneva

The photodesorption yield i.e. the number of desorbed molecules per incident photon is a rather straightforward measurement at room temperature. However, when the vacuum chamber is at cryogenic temperature, the desorbed molecules will be physisorbed on the cold surface, especially if the vacuum chamber is a virtually closed volume like in the Large Hadron Collider (LHC), the next CERN particle accelerator. These physisorbed molecules may, in turn, be recycled into the vacuum chamber by the photodesorption process. As the physisorbed molecules are much weaker bound than the chemisorbed molecules, the recycling photodesorption yield is expected larger than the photodesorption yield. The knowledge of this recycling photodesorption yield at cryogenic temperature is essential for the vacuum and hence for LHC beam lifetime. By using different experimental methods, it was possible to demonstrate the evidence of recycling photodesorption yield as well as photocracking of physisorbed molecules. This yield is found, assuming a unity sticking coefficient, to range from 10E-3 for heavier gases to 1 molecule / photon for lighter gases at one monolayer coverage