Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy are used in this study to investigate low-defect and defective MgO(100) thin films. Unlike low-defect films, defective films exhibit a new spectroscopic feature located similar to2 eV above the top of valence band. Exposing the defective film to oxygen quenches the emission of electrons from F centers created on the surface and in the subsurface regions. Extended defects, unseen in the MIES spectra of the clean surface, are detectable using NO titration. MIES and thermal programmed desorption indicate that at similar to 100 K NO adsorbs dissociatively on defects, forming N2O. Only a small fraction of the MgO surface becomes covered with N2O at similar to 100 K for the low-defect MgO film indicating that N2O molecules preferentially adsorb on the extended defects. The saturation coverage of N2O increases appreciably for the defective sample.