Positive Ion Emission Accompanying UV Irradiation of Single Crystal MgO and NaNO 3
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Although MgO is much more resistant to radiolysis by 248-nm photons than NaNO 3, the ion emission processes at low fluence have much in common: both materials yield high energy ions (> 5 eV kinetic energy) with a strongly nonlinear fluence dependence. We report time-of-flight measurements of quadrupole mass-selected Mg' from polished,
single crystal MgO and Na' from cleaved, single crystal NaNO3 . At fluences between 10 2
and 1000 mJ/cm , the Mg' intensities show a strongly nonlinear fluence dependence 2
which decreases to roughly second order behavior at fluences above 100 mJ/cm . The Na' intensities display third or fourth order emission kinetics throughout the experimental
range of fluences. We attribute these emissions to cations adsorbed atop surface electron traps where the cation is ejected when the underlying trap is photo-ionized. The potential energy of this defect configuration accounts for the observed ion kinetic energies. However, the direct photo-ionization of surface vacancy/adsorbed ion defects with 5 eV photons should not be possible. Thus we propose that emission requires the photoionization of nearby electron traps, followed by photo-induced charge transfer to the empty traps. We show that a sequence of single-photon absorption events [involving photo-ionization, charge transfer, and electron retrapping] accounts for the strongly
nonlinear fluence dependence. INTRODUCTION In recent work,1 we have shown that Mg' and Mg2" photodesorbed from MgO during
low-fluence laser irradiation at 248 nm can have surprisingly high energies-up to 20 eV for 2
Mg ,. Time-of-flight (TOF) measurements of quadrupole mass-selected ion emissions show stable energy distributions over a significant range of fluences. In the case of MgO, several lines of evidence point to an electrostatic emission mechanism: the kinetic energies of the doubly charged species are considerably higher than the kinetic energies of the singly charged species; further, the total ion emissions are strongly directed along the surface normal.' The emission intensities at a given fluence were strongly dependent on treatments known to affect the surface defect density. We have attributed these emissions to cations adsorbed on the surface at sites above occupied electron traps; emission occurs when the laser photo-ionizes the underlyinq electron trap. This process is similar to the Knotek-Feibelman mechanism for photodesorption, except that the Knotek-Feibelman mechanism does not require lattice defects. Lattice defects allow for photodesorption at much lower photon energies (here 5 eV as opposed to the hundreds of eV required for core-hole production).3'4 Defect-related emissions also permit the desorption of cations, whereas the Knotek-Feibleman mechanism yields anions only. In this work, we extend the Mg' TOF measurements to lower fluences by applying sensitive pulse counting techniques to the mass-selected ion signal. We show that the emission intensities are strongly nonlinear functions of fluence in this fluence range. Similar measuremen
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