Figure 4

C 1s spectra measured at hv = 750 eV (circles) while the sample is biased to +30 V. Curve fits (red solid lines) are also given. In all cases, the sample is kept at +2 °C. The water pressure is 0.5 mbar (RH = 7%). Pyridine gas is added (partial pressures are indicated, from 0.1 to 0.5 mbar) while the water pressure is kept constant. The UHV pressure is 10−8 mbar. After pumping down the gas mixture, a pressure of 10−7 mbar is recovered. All spectra are aligned with respect to the Mg 2p maximum at 50.00 eV and the O 1s maximum at 532.32 eV (see text and Supporting Information, section S2). We recall to the reader that in biased sample conditions the gas phase contribution is eliminated (see Methods).