Photon stimulated desorption of an unbaked stainless steel chamber by 3.75keV critical energy photons

A stainless steel vacuum chamber, which had been going through a usual labo ratory ultrahigh vacuum process, was exposed to a white synchrotron radiati on photon beam, 3.75 keV critical energy, 70 W power at DCI. The vessel was however not baked "in situ" in order to test the necessity of this interme diate step. This is an issue with far reaching consequences for new synchro tron radiation sources. Beam scrubbing from a photon beam dose of 2.3 x 10( 23) photons m(-1) was found to decrease the molecular yields from an initia l value of 10(-2) molecules per photon to below 10(-5) for the main species CO and CO2. The cleaning effect of the secondaries was clearly demonstrate d, thus justifying the definition of the photon beam dose. It also explains the strong reduction of the static pressure. Wall pumping developed by the irradiation of the 3.6 m long tube was found to develop a pumping speed of respectively, 750 and 1900 l s(-1) for CO and CO2 and nil for H-2, CH4, an d N-2. The resulting pumping capacity amounts to an equivalent of 10(-4) mo nolayer for both CO and CO2 to be compared to a total of 2 monolayers remov ed by beam scrubbing. These results are finally applied to predict the gas beam lifetime for a new synchrotron radiation source SOLEIL. It appears tha t a reasonable Lifetime can be reached in a short time in the absence of in situ bakeout. (C) 1999 American Vacuum Society. [S0734-2101(99)00302-4].