On surface coatings and secondary yield of Nb3Sn and Nb

In rf cavities, Nb3Sn and Nb surfaces show features that have been blamed on unavoidable oxide layers. In addition to the oxidation, physi- and chemisorption take place, which strongly influence the work function and the secondary yield. To measure quantitatively the oxide and sorption layers, x-ray photoelectron spectroscopy (XPS) measurements have been carried out for Nb surfaces subjected to the typical procedures, such as oxipolishing (OP), ultra high vacuum annealing (HT), handling in ’’vacuum,’’ air, H2O, H2O2, CH3OH, or impact of ions and electrons. The XPS results show that all Nb3Sn and Nb surfaces used are coated with a dielectric oxide layer thicker than 2 nm, saturating in growth at 6 nm. These oxides are coated with a sorption layer containing O and C of at least 2 monolayer equivalent (ML), but as much as 10 ML have been observed. The excited states of the oxides and the excited localized states of the Nb2O5-H2O chemisorption state enhance the secondary electron emission, whereas the physisorbed hydrocarbons show enhanced absorption of slow electrons, especially after electron or x-ray induced ’’conditioning.’’ The enhanced secondary emission from the Nb2O5-H2O state positively charges the dielectric, lowering the work function, whereas the enhanced electron absorption of ’’conditioned’’ hydrocarbons negatively charges this region and increases the work function.