Influence of chemical structure and beam degradation on the kinetic energy of molecular secondary ions in keV ion sputtering of polymers

The secondary ions sputtered from thin polymer films under 15 keV, Ga+ bombardment were mass- and energy-analysed by means of a Time-of-Flight Secondary Ion Mass Spectrometer. The comparison between aliphatic (polyisobutylene) and aromatic polyolefins (polystyrene) allows to propose a process of ion emission in which the mean kinetic energy of the secondary ions is related the degree of fragmentation with respect to the polymer target structure. The important yield decrease and the broadening of the kinetic energy distributions (KEDs) observed for the CxH2x ions sputtered from pre-bombarded polyisobutylene (10(12)-10(15) ions/cm(2)) are explained in terms of physico-chemical modification of the surface. The fast metastable decay of molecular ions in the acceleration section of the spectrometer is also investigated. (C) 1998 Elsevier Science B.V.