Figure 2
(a) Sr 3d and Mg 2p spectra (small crosses) measured at hν = 750 eV with a sample bias of +30 V (the binding energies are corrected). The Sr 3d spectrum is fitted with a 3d3/2/3d5/2 doublet (red solid line). The Mg 2p level is also a doublet but because of the small spin-orbit splitting energy (0.28 eV), it is fitted by a single component. (b) O 1s spectra (small crosses) of the grounded sample measured at hν = 750 eV. Curve fits (red solid line) are also given. The grey component corresponds to the clay lattice oxygen and to confined water, while the blue one corresponds to gas phase water. 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. All spectra are aligned with respect to the Mg 2p maximum at 50.00 eV and the O 1s maximum (clay) at 532.32 eV (see text and Supporting Information, section S1). We recall to the reader that the gas-phase contribution can only be seen when the sample is grounded (see Methods).
