ENERGIES AND YIELDS OF SECONDARY IONS EJECTED FROM ALKALI-HALIDES BY A 0.18-6.0 MEV NITROGEN BEAM

Nuclear and electronic sputtering was studied for the alkali halides L iF, CsI, NaF, NaCl, NaBr and NaI being vacuum evaporated on thin Al-fo ils and irradiated by N-14 ions having an energy loss of 20-190 eV/Ang strom. A double-grid time-of-flight technique was used to determine th e yields and the axial energy distributions of the alkali and halide i ons ejected from the sample surface. In case of LiF, the most carefull y investigated compound, the dependence of ion yields on the energy lo ss 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 m ean axial energies were found to range from 0.26 to 2.0 eV, their ener gy 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 wit h the charge state of the incident ions. These observations lead to th e general conclusion, that electronic sputtering from alkali halides i s established by cooperative processes and that probably electron-hole production in deep layers contribute essentially to formation and eje ction of ions at the surface. The results are discussed in terms of ex citon production and migration.