The XPS measurements were carried out in a commercial UHV system (SPECS) with a background pressure of around 2  10 -10 mbar, which among others was equipped with an Xray source (SPECS XR50, Al-Kα and Mg-Kα) and a hemispherical analyzer (SPECS, DLSEGD-Phoibos-Has3500). We used non-monochromatized Al-Kα radiation (1486.6 eV) with the X-ray gun operated at a power of 250 W (U = 14 kV, I = 17.8 mA). The XP spectra were recorded at a pass energy Epass of 100 eV at grazing emission (80° to the surface normal, surface sensitive mode). For the quantitative analysis of the XP spectra, fitting of the peaks was performed using the Igor pro 8.04 software with a simultaneous fit of the background (Shirley + slope) and signal, assuming a pseudo-Voigt type peak shape, which is a linear combination of a Gaussian and a Lorentzian function. Atomic sensitivity factors (ASFs) for our set-up for the different spectral regions were evaluated using a molecularly adsorbed BMP-TFSI (C11H20F6N2O4S2) multilayer film on the graphite (0001) substrate, where the ASF for F 1s is set to 1, which leads to ASFs of C1s: 0.21, O1s: 0.57, S 2p: 0.40 and N 1s: 0.35. The highly oriented pyrolytic graphite (0001) (HOPG) substrate was purchased from MaTeck (ZYA, mosaic spread 0.4°± 0.1°, size of 10 mm  10 mm  < 1 mm). Before transferring the HOPG into the load lock of the UHV system, it was fixed on a tantalum sample plate with conductive silver paste, heated in a N2 flooded oven for 30 minutes at 720 K to degas the silver paste, and then freshly cleaved. Afterwards, it was transferred from the load lock into the UHV chamber and flashed to ∼ 600 K to generate a clean graphite (0001) surface. The sample temperature in the main UHV chamber was measured via a pyrometer (LumaSense Technologies, IP 140). The ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) was filled into a quartz crucible, which was mounted in a Knudsen effusion cell (Ventiotec, OVD-3). To generate a pure, water-free IL, it was first degassed in the load lock system at around 400 K. Subsequently, the IL source was heated up in the main UHV chamber to the IL evaporation temperature of 450 K, where evaporation started, but with a negligible increase of the base pressure was. Under these conditions, the IL deposition rate was approximately 0.1 ML min , with 1 monolayer (ML) defined as a layer at saturation coverage in the first layer (approx. layer thickness d: 3.5 Å).