Fig. 4: Comparison of one-hole spectra of H₂O, Na⁺, and Mg²⁺.

From the top, liquid-water PES spectrum recorded using a photon energy of 532 eV, i.e., under relatively bulk-sensitive conditions⁵¹ (blue), Na IRD spectrum (red dots, experiment, and red dashed, theory), Mg IRD spectrum (green dots, experiment, and green dashed, theory). Experimental and theoretical IRD spectra are enlargements of the data shown in Fig. 2. Solid black curves indicate a fitting of the experimental data using a minimal number of Gaussian curves. Original theoretical data for Na and Mg are represented by black bars, and the dashed curves for their convolution with a Gaussian function (FWHM = 0.8 eV) to facilitate the comparison with the experimental spectra. Vertical solid lines indicate calculated binding energies from Ref. ¹⁴.