Supplemental Material, six supplemental Figures plus text and Figure captions

To support the findings described in the main article, we present pressure changes due to the release of contaminations into the gas phase during the laser melting in Figure S1, and additional SEM/EDS experiments of Carbon-containing particulates detected in the laser-polished Nb foil in Figure S2). A simulation of the escape depth for the nitrogen, oxygen and niobium signals during EDX analysis is provided in Figure S3. Figure S4 compares X-ray absorption spectra of an untreated Nb foil to a laser-polished foil under a nitrogen pressure of 0.1 mbar, and a niobium nitride reference spectrum, and in Figure S5, results of supplemental X-ray diffraction studies of the raw Nb foils and the laser-polished foil in N2-atmosphere are presented, to prove the preservation of the original bcc-structure of the niobium and absence of e.g. Nb nitrides after laser polishing in N2-atmospheres, in agreement with the X-ray absorption data. Finally, a calculation of the size for the heat-affected zone during the laser polishing treatment is provided in Figure S6.