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Nuclear and electronic sputtering was studied for the alkali halides LiF, CsI, NaF, NaCl, NaBr and NaI being vacuum evaporated on thin Al-foils and irradiated by 14 N ions having an energy loss of 20–190 eV/A. A double-grid time-of-flight technique was used to determine the yields and the axial energy distributions of the alkali and halide ions ejected from the sample surface. In case of LiF, the most carefully investigated compound, the dependence of ion yields on the energy loss and the shape of the energy distributions clearly mark the onset of collisional sputtering towards low beam energies. Apart from F − , for all other investigated alkali and halide ions electronic sputtering is the dominant ejection process. Above 1 MeV bombarding energy, their mean axial energies were found to range from 0.26 to 2.0 eV, their energy distributions fall off with E − n , n having values between 3.0 and 4.5. In cases, where the nuclear energy loss is relatively low, the ion yields scale with about the second power of the electronic energy loss; the mean axial energies and the yields change only little or not with the charge state of the incident ions. These observations lead to the general conclusion, that electronic sputtering from alkali halides is established by cooperative processes and that probably electron-hole production in deep layers contribute essentially to formation and ejection of ions at the surface. The results are discussed in terms of exciton production and migration. |
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