Effects of potential energy on sputtering ofF+from theCaF2(111)surface by noble-gas ion bombardment

Abstract

The mechanism of ion- and electron-stimulated desorption of ${\mathrm{F}}^{+}$ from the ${\mathrm{CaF}}_{2}(111)$ surface has been investigated. By ${\mathrm{He}}^{+}$ bombardment, a considerable amount of the ${\mathrm{F}}^{+}$ ions are sputtered due to the potential energy of ${\mathrm{He}}^{+}.$ The yield of ${\mathrm{F}}^{+}$ via the potential sputtering from the polycrystalline ${\mathrm{CaF}}_{2}$ film is three times as large as that from the ${\mathrm{CaF}}_{2}(111)$ surface, suggesting that the fluorine at the lower coordination sites or the defect sites preferentially desorbs as ${\mathrm{F}}^{+}.$ The probability for the potential sputtering increases relative to that for the kinetic sputtering with increase of the primary energy of ${\mathrm{He}}^{+},$ whereas most of the ${\mathrm{F}}^{+}$ ions by ${\mathrm{Ar}}^{+}$ are kinetically sputtered. On the basis of these experiments as well as ab initio molecular-orbital calculations, it is concluded that desorption of fluorine is initiated by creation of the F $2s$ hole as a result of the nonadiabatic charge exchange with the primary ion. The ionization of the core-excited fluorine occurs on the way out from the surface due to the intra-atomic Auger decay. The fluorine can be ionized positively during collision with He backscattered from the solid due to the electron promotion mechanism, that is responsible for the kinetic sputtering of ${\mathrm{F}}^{+}.$