RIXS measurements were carried out at room temperature and under 10 -8 mbar at U41-PEAXIS at BESSY II, Germany. The spectrometer was positioned at specular conditions relative to 60 degree scattering angle and was optimized to a combined resolution of 90 meV using a carbon tape. The excitation energy of each 1D curve was at the O K-edge at 531 eV, and the acquisition time was 20 min.