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Abstract

The formation of surface color centers (FS centers) by electron bombardment of thin MgO(001) films is investigated using electron paramagnetic resonance and low-temperature scanning tunneling microscopy. At low electron doses both techniques indicate the formation of singly occupied color centers (FS+), whereas at high electron doses the doubly occupied type (FS0) is dominant. It is suggested that with increasing electron dose FS+ centers are transformed into FS0. Tunneling spectra of individual FS0 centers reveal a large distribution of energetic positions of occupied and unoccupied states, which is caused by local variations of the coordination number of the defects and explains the broad signals usually detected with integrating spectroscopic techniques.