MeV ion sputtering of polymers: correlation between secondary ion radial velocity distributions and heavy ion track structure

Abstract

Systematic investigations of the initial radial velocity distributions of low mass positive and negative secondary ions, sputtered electronically from thin films of polyvinylidene fluoride and polystyrene, are reported. 72.3 MeV 127I13+ primary ions bombard the targets at 45° angle of incidence. Sputtered secondary ions in an individual MeV ion impact are analysed in a high resolution time-of-flight mass spectrometer. The accurate mass measurements of all ion peaks in the range from 1 to 100 m/z provide unequivocal determination of the chemical composition of these ions, forming homologous series, CnH±m and CnHmF±p. Plots of both the initial mean radial velocity (〈vx〉) and kinetic energy (α 〈v2x〉) as a function of the ion m/z results in a periodic pattern. Ions with lower hydrogen content exhibit wider velocity distributions (i.e. higher 〈v2x〉) and (〈vx〉) directed towards the primary ion trajectory. Ions with higher hydrogen content have lower mean kinetic energies and 〈vx〉 directed away from the incident ion trajectory. We argue that the 〈vx〉 and 〈v2x〉 periodic behaviour, connected to the chemical constitution of the ions, reflects the radial profile of the deposited energy density in the heavy ion track.