Non-equilibrium emission of secondary ions from BeO films sputtered by swift gold ions

Electronic sputtering of thin BeO films induced by Au26+ and Au41+ ions has been studied at the beam energy of 1.8 MeV/u and compared to the experimental results obtained under nuclear sputtering conditions. Energy spectra and angular distributions of secondary positive ions (Be+ and O+) as well as negative ions (O-, BeO- and BeO2-) have been measured using a quadrupole mass spectrometer. Contrary to the nuclear sputtering, the data for swift heavy-ion irradiation show significant differences in the sputtering process between positive and negative ions. Whereas the angular distribution of the negative ions is nearly isotropic, the emission of the positive ions is strongly anisotropic and enhanced at the directions corresponding to the incident half-space. Additionally, an enhanced high-energy contribution in the energy spectra of positive ions can be observed. The electronic sputtering process of positive ions cannot be explained in the frame of the simple thermal spike model assuming a thermal equilibrium within the ion-track in the target material. An application of the gas-flow model describing an adiabatic expansion of high temperature plasma outside the target has been proposed. (C) 2004 Elsevier B.V. All rights reserved.