Positive ion emission accompanying UV irradiation of single crystal MgO and NaNO3

Although MgO is much more resistant to radiolysis by 248-nm photons than NaNO3, 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. The authors 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 and 1,000 mJ/cm2, the Mg+ intensities show a strongly nonlinear fluence dependence which decreases to roughly second order behavior at fluences above 100 mJ/cm2. The Na+ intensities display third or fourth order emission kinetics throughout the experimental range of fluences. The authors 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 the authors propose that emission requires the photoionization of nearby electron traps, followed by photo-induced charge transfer to the empty traps. They show that a sequence of single-photon absorption events [involving photo-ionization, charge transfer, and electron retrapping] accounts for the strongly nonlinear fluence dependence