Interaction of wide band gap single crystals with 248 nm excimer laser radiation. IV. Positive ion emission from MgO and NaNO3
Ermer, D. R.
Langford, S. C.
Hipps, K. W.
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We report quadrupole mass-selected, time-of-ﬂight measurements of Mg1 from polished, single crystal MgO and Na1 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 Mg1 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. © 1996 American Institute of Physics. [S0021-8979~96!07123-X].