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Abstract Surface structures on radio-frequency (RF) superconductors are crucially important in determining their interaction with the RF field. Here we investigate the surface compositions, structural profiles, and valence distributions of oxides, carbides, and impurities on niobium (Nb) and niobium–tin (Nb₃Sn)in situunder different processing conditions. We establish the underlying mechanisms of vacuum baking and nitrogen processing in Nb and demonstrate that carbide formation induced during high-temperature baking, regardless of gas environment, determines subsequent oxide formation upon air exposure or low-temperature baking, leading to modifications of the electron population profile. Our findings support the combined contribution of surface oxides and second-phase formation to the outcome of ultra-high vacuum baking (oxygen processing) and nitrogen processing. Also, we observe that vapor-diffused Nb₃Sn contains thick metastable oxides, while electrochemically synthesized Nb₃Sn only has a thin oxide layer. Our findings reveal fundamental mechanisms of baking and processing Nb and Nb₃Sn surface structures for high-performance superconducting RF and quantum applications. 

We have measured the absolute doubly differential angular sputtering yield for 20 keV Kr+ impacting a polycrystalline Cu slab at an incidence angle of θi = 45° relative to the surface normal. Sputtered Cu atoms were captured using collectors mounted on a half dome above the sample, and the sputtering distribution was measured as a function of the sputtering polar, θs, and azimuthal, ϕs, angles. Absolute results of the sputtering yield were determined from the mass gain of each collector, the ion dose, and the solid angle subtended, after irradiation to a total fluence of ∼1 × 1018 ions/cm2. Our approach overcomes shortcomings of commonly used methods that only provide relative yields as a function of θs in the incidence plane (defined by the ion velocity and the surface normal). Our experimental results display an azimuthal variation that increases with increasing θs and is clearly discrepant with simulations using binary collision theory. We attribute the observed azimuthal anisotropy to ion-induced formation of micro- and nano-scale surface features that suppress the sputtering yield through shadowing and redeposition effects, neither of which are accounted for in the simulations. Our experimental results demonstrate the importance of doubly differential angular sputtering studies to probe ion sputtering processes at a fundamental level and to explore the effect of ion-beam-generated surface roughness. 

The lunar exosphere is generated by a variety of processes: photodesorption from solar UV radiation (PSD), solar wind ion sputtering, meteoritic bombardment, radioactive decay, and thermal desorption. While remote or orbital temporal measurements provide in situ clues to source mechanisms, individual ejection processes are more easily and deeply investigated in laboratory experiments on returned Apollo samples and analogs, allowing quantitative comparisons at lunar-like pressures and temperature. The importance of laboratory experiments cannot be overemphasized, providing measurements of ejection probabilities relevant to exospheric formation, as well as metrics such as surface charge, surface composition and phase, and meteoritic-impact plume characterization. These parameters can be convolved to describe telescopic observations as well as phenomena observed at the lunar surface by orbital / lander measurements, providing ground truth for models of spatial and temporal variations in the exosphere. The following discussion of laboratory work pertinent to the generation of the lunar atmosphere is a starting point for those interested in laboratory simulations and is by no means an exhaustive review.