Inferring ejection distances and a surface energy profile in keV particle bombardment experiments

The disappearance cross-sections and kinetic energy distributions of fragment ions sputtered from polystyrene thin films under 12 keV ⁶⁹Ga⁺ ion bombardment are measured using a time-of-flight spectrometer. Even though the disappearance cross-sections are often used as an indicator of radial ion beam damage, the derived radii range from 2 to 10 angstroms depending on the particular hydrocarbon fragment. Therefore, the disappearance cross-sections cannot be directly related to a single global quantity of damage per incident particle. Likewise, the widths of the measured kinetic energy depend on the particular hydrocarbon fragment. Both the disappearance cross-sections and the widths of the kinetic energy distribution superficially relate to fragment mass but the correlation is not perfect. We develop a hypothesis that the disappearance cross-section and the width of the kinetic energy distribution actually correlate with the ejection radius of the particular fragment. Thus, the kinetic energy distributions provide an estimate of the radial extent of the energy density in the ejection region. Our interpretations are supported by molecular dynamics simulation results. For comparison with previously reported data, our results indicate that the deposited energy profile is 4-5 times narrower than for 72.3 MeV, electronic sputtering of PVDF.