THE CHEMICAL ORIGIN OF SEY AT TECHNICAL SURFACES

The secondary emission yield (SEY) properties of colaminatedCu samples for LHC beam screens arecorrelated to the surface chemical compositiondetermined by X-ray photoelectron spectroscopy. Thesurface of the "as received" samples is characterized bythe presence of significant quantities of contaminatingadsorbates and by the maximum of the SEY curve (dmax)being as high as 2.2. After extended electron scrubbing atkinetic energy of 10 and 500 eV, the dmax value drops tothe ultimate values of 1.35 and 1.1, respectively. In bothcases the surface oxidized phases are significantlyreduced, whereas only in the sample scrubbed at 500 eVthe formation of a graphitic-like C layer is observed.We find that the electron scrubbing of technical Cusurfaces can be described as occurring in two steps, wherethe first step consists in the electron induced desorption ofweakly bound contaminants that occurs indifferently at 10and at 500 eV and corresponds to a partial decrease ofdmax, and the second step, activated by more energeticelectrons and becoming evident at high doses, whichincreases the number of graphitic-like C-C bonds via thedissociation of adsorbates already contaminating the "asreceived" surface or accumulating on this surface duringirradiation. Our results demonstrate how the kineticenergy of impinging electrons is a crucial parameter whenconditioning technical surfaces of Cu and other metals bymeans of electron induced chemical processing.