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 200–1100 mJ/cm2 were determined by combined quadrupole mass spectrometry and time-of-flight techniques. The dominant ionic species is Mg+, although small amounts of Mg2+, MgO+, and Mg2O+ are also observed. In particular, the Mg+ and Mg2+ energies distributions each show two peaks, with the energies of the Mg2+ peaks at significantly higher energies. Ion trajectory simulations (accounting for Coulomb forces only and assuming no surface relaxation) suggest that Mg2+ and Mg+ adsorbed at sites directly atop surface F-centers (oxygen vacancies with two trapped electrons) would be ejected upon photo-ionization of the F-center. The experimentally observed kinetic energies and angular distributions agree well with the energies and angles predicted by these simulations.