SELF-TRAPPING HOLES AND EXCITONS IN THE BULK AND ON THE (100) SURFACES OF MGO

Self-trapping of holes and excitons in the bulk and on the surface of MgO has been simulated using the self-consistent semi-empirical intermediate neglect of differential overlap method within an embedded molecular cluster model. Our results suggest that, while holes are not self-trapped in the bulk of MgO, they probably are on the surface. It is likely that excitons in both the bulk material and on the (100) surface are self-trapped but that they are highly mobile. The 'on-centre' model of bulk exciton self-trapping yields a Stokes shift of 0.65 eV which is in agreement with experiment. Similarly, the calculated peak energy of 6.4 eV for the excitonic excitation spectrum of the (100) surface agrees with the experimental reflectance peak value of 6.6 eV. The emission energy of the self-trapped exciton on the surface is determined to be 5.7 eV. The results of our calculations show that a single excitonic excitation cannot lead to surface disintegration. However, with high-density irradiation, excitation of an F+-O- metastable state, derived from an exciton, may cause surface disintegration and oxygen atom emission.