Changes in the ionization potential of MgO and CaO film surfaces upon heating in air and vacuum, which are necessary processes in the manufacture of plasma display panels, were investigated by metastable deexcitation spectroscopy. Heating at 500 degrees C for 1 min in vacuum is effective for cleaning an MgO film surface previously heated in air, resulting in the recovery of the ionization potential of the cleaned MgO surface from the increased value upon heating in air. A similar treatment is not effective for the CaO surface. This difference in behavior can be attributed to the stronger bonding states of H2O and CO2 on CaO than those on MgO. This is consistent with the fact that CaO has not yet been practically used as a protective layer in plasma display panels. Furthermore, the firing voltages of the plasma predicted for the MgO and CaO protective layers from the ionization potentials determined by metastable de-excitation spectroscopy were found to qualitatively agree with measured firing voltages obtained for a test panel. The present results confirmed that metastable de-excitation spectroscopy is effective for evaluating protective layers used in plasma display panels. (C) 2012 Elsevier B. V. All rights reserved.