ELECTRONIC SPUTTERING OF FULLERENES AND THE INFLUENCE OF PRIMARY ION CHARGE-STATE

Even-numbered positive ion carbon clusters, fullerenes (C2n, n = 20, 21, . . . ), are formed and ejected as a result of the interaction of swift atomic ions with a solid film of organic polymer - poly(vinylidene difluoride), PVDF. Studies of that phenomenon including the dependence of fullerene yield on the incident MeV ion's stopping power and initial radial velocity distributions of ejected ions, have been reported earlier [G. Brinkmalm et al., Chem. Phys. Lett. 191 (1992) 345; Phys. Rev. B 47 (1993) 7560]. In this paper, the dependence of the yield of the ejected cluster ions from PVDF as a function of the charge state of the incident MeV ion is reported. Swift 72.3 MeV I-127 ions in charge states ranging from + 13 to + 25 from the Uppsala EN-tandem accelerator are used in the experiments, and the secondary ion yield is measured in a time-of-flight mass spectrometer, equipped with an electrostatic ion mirror. The results are compared to data for secondary ions, ejected by the same primary ions, from samples of synthetic fullerenes (C60 and C70). No dependence of the yield on the charge state of the incoming ion is observed for the high mass even-numbered carbon clusters from PVDF, while yields of C60 and C70 ions from synthetic fullerene targets show a weak charge state dependence. This finding indicates that excited polymer material from layers deeper than the MeV ion charge equilibration length in the solid contributes to the formation of ejected carbon cluster ions ejected from PVDF.