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The Chemical Origin of SEY at Technical Surfaces

The secondary emission yield (SEY) properties of colaminated Cu samples for LHC beam screens are correlated to the surface chemical composition determined by X-ray photoelectron spectroscopy. The surface of the "as received" samples is characterized by the presence of significant quantities of contaminating adsorbates and by the maximum of the SEY curve (dmax) being as high as 2.2. After extended electron scrubbing at kinetic energy of 10 and 500 eV, the dmax value drops to the ultimate values of 1.35 and 1.1, respectively. In both cases the surface oxidized phases are significantly reduced, whereas only in the sample scrubbed at 500 eV the formation of a graphitic-like C layer is observed. We find that the electron scrubbing of technical Cu surfaces can be described as occurring in two steps, where the first step consists in the electron induced desorption of weakly bound contaminants that occurs indifferently at 10 and at 500 eV and corresponds to a partial decrease of dmax, and the second step, activated by more energetic electrons and becoming evident at high doses, which increases the number of graphitic-like C-C bonds via the dissociation of adsorbates already contaminating the "as received" surface or accumulating on this surface during irradiation. Our results demonstrate how the kinetic energy of impinging electrons is a crucial parameter when conditioning technical surfaces of Cu and other metals by means of electron induced chemical processing.