Radial velocity distributions of secondary ions ejected from PTFE by MeV atomic ions

Abstract

In this paper, systematic investigations of the initial radial velocity distributions of CnFm+ secondary ions sputtered by 72.3 MeV 127I+13 ions from polytetrafluoroethylene (PTFE) foils are presented. The initial radial velocity distributions are obtained by monitoring the secondary ion yield as a function of the voltages applied to two sets of deflection plates installed parallel to the ion optical axis of a time-of-flight mass spectrometer. The mean radial velocity, 〈vx〉, of CnFm+ ions (1 < n < 6and0 < m < 13) depends on the number of fluorine atoms, showing a periodic behaviour when plotted as a function of the ion mass. Ions with low fluorine content have 〈vx〉 directed towards the primary ion's line-of-incidence (positive mean velocities). Ions with a large number of fluorine atoms are ejected in an off-normal direction away from the incoming MeV ion trajectory (negative mean velocities). The ion emission can not be explained only by evaporative type processes. Mechanisms involving correlated momentum transfer e.g. pressure pulse or shock wave induced ejection, should also be taken into account. The data obtained for fluorocarbon ions from a purely fluorinated compound generalise a similar effect observed previously for CnHm+ ions, sputtered from different hydrogen containing polymer targets [R.M. Papaléo et al., Nucl. Instr. and Meth. B 91 (1994) 667].