Formation of fullerenes in MeV-ion sputtering from organic solids

Even-numbered positive-ion carbon clusters (C₂ₙ, n=20,21,...) are ejected from a solid film of organic polymer---poly(vinylidene difluoride) (PVDF)---when bombarded by MeV atomic ions. Carbon-cluster ions are formed as a result of a single primary-ion impact. The distribution of cluster sizes suggests that the clusters have closed carbon-cage (fullerene) structure. Measurements of the yield and initial-velocity distributions of the ejected cluster ions are performed in a time-of-flight mass spectrometer equipped with an electrostatic ion mirror. The results are compared to data for ions ejected by MeV projectiles from films of synthetic fullerene (C₆₀ and C₇₀) targets. The initial-radial-velocity distribution for cluster ions ejected from the polymer differ markedly from those of synthetic fullerene targets. While C₆₀ and C₇₀ ions from synthetic fullerene targets have ejection-angle distributions symmetric with respect to the target-surface normal, the mean takeoff angle of carbon-cluster ions ejected from PVDF is off-normal and towards the direction of the incoming primary ion. It is also a clear indication that carbon-cluster ions ejected from PVDF originate in the high-energy-density region created by the fast primary ion. Experiments for establishing the dependence of the carbon-cluster yield Y on the energy loss of the primary ion have been performed. A stronger dependence of the yield for the C₆₀ ions desorbed from PVDF targets [Y{∝}(dE/dx)3.4] than for ions from targets with synthetic C₆₀ [Y{∝}(dE/dx)2.0] has been observed. This supports the conclusion that carbon-cluster ions from PVDF are formed in the axially expanding high-energy-density plasma region.