THE ROLE OF DEFECTS IN THE LASER-ABLATION OF WIDE BANDGAP MATERIALS

In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent), the absorption of laser radiation at 248 nm (5 eV photons) at modest fluences is defect dominated. We describe a technique for imaging the initial defect densities by their luminescence at low laser fluences and apply it to cleaved MgO. High defect densities are observed along many cleavage steps, consistent with previous observations of strong point-to-point variations in the ablative response of cleaved MgO surfaces. The composition of neutral particles emitted from the surface can be strongly influenced by defects (here, impurities), as shown by the intense emission of carbon oxides and the correspondingly weak emissions of atomic and molecular oxygen from arc-fused MgO at fluences below that required for sustained emission. We also present evidence for defect-mediated ion emission at these low fluences. Mass selected time-of-flight measurements of Mg + emission show a well defined kinetic energy in the range of 4-5 eV, consistent with electrostatic ejection of weakly bound ions following the ionization of a nearby surface F-center.