THE ROLE OF DEFECTS IN LASER-INDUCED POSITIVE-ION EMISSION FROM IONIC-CRYSTALS

The energy distributions of positive ions produced by exposing single- crystal MgO to pulsed 248 nm excimer laser light at fluences of 200-11 00 mJ/cm(2) were determined by combined quadrupole mass spectrometry a nd time-of-flight techniques. The dominant ionic species is Mg+, altho ugh small amounts of Mg2+, MgO+, and Mg2O+ are also observed. In parti cular, the Mg+ and Mg2+ energies distributions each show two peaks, wi th the energies of the Mg2+ peaks at significantly higher energies. Io n trajectory simulations (accounting for Coulomb forces only and assum ing no surface relaxation) suggest that Mg2+ and Mg+ adsorbed at sites directly atop surface F-centers (oxygen vacancies with two trapped el ectrons) would be ejected upon photo-ionization of the F-center. The e xperimentally observed kinetic energies and angular distributions agre e well with the energies and angles predicted by these simulations.