Interaction of wide band gap single crystals with 248 nm excimer laser radiation. IV. Positive ion emission from MgO and NaNO3

Abstract

We report quadrupole mass-selected, time-of-flight measurements of Mg+ from polished, single crystal MgO and Na+ from cleaved, single crystal NaNO3 exposed to 248 nm (5 eV) laser radiation. A large fraction of the ions emitted from these materials have energies well above the energy of the incident photon. As the fluence is raised from low values, the ion intensities show thresholdlike behavior with a high-order fluence dependence (roughly sixth order). At still higher fluences, the fluence dependence of Mg+ from MgO decreases to roughly second order. We attribute these emissions to weakly bound ions adsorbed atop surface electron traps; when the underlying vacancy is photoionized, the adsorbed ion is electrostatistically ejected at high energy. We argue that several photons are required to ionize a surface electron trap beneath an adsorbed ion, accounting for the high-order fluence dependence and satisfying conservation of energy. (Several 5 eV photons are required to produce a 10 eV ion.) We show that a sequence of single-photon absorption events involving photoionization, charge transfer, and retrapping account for this unusual fluence dependence. These emission intensities are strong functions of surface treatments which increase defect densities, e.g., abrasion in the case of MgO and electron bombardment in the case of NaNO3.