Changes in ionization potentials of MgO and CaO films upon heating in air and vacuum investigated by metastable de-excitation spectroscopy

► Chemisorption of H2O and CO2 markedly changes the ionization potential of CaO. ► Heating in vacuum is effective to cleaning the MgO surface but not effective for CaO. ► The firing voltage of plasma display panels and the ionization potential correlate. ► Metastable de-excitation spectroscopy is effective to evaluating protective layers.