ToF-SIMS analyses of polystyrene and dibenzanthracene: Evidence for fragmentation and metastable decay processes in molecular secondary ion emission

To understand the sputtering processes in unsaturated polymers, dibenzanthracene and polystyrene samples were bombarded by 15 keV, Ga+ ions and the secondary ions were mass- and energy-analyzed by means of a time-of-flight spectrometer. The influence of various phenomena, which may play an important role in the secondary ion emission from polystyrene surfaces, is investigated: (1) emission of ''original'' fragments, reflecting the polymer structure; (2) emission with fragmentation, rearrangements or hydrogen losses; (3) formation of molecular ions by hydrogen or proton capture; and (4) unimolecular dissociation of the secondary ions. The dibenzanthracene molecule (C22H14) was chosen as a model compound for the study of the emission and metastable decay of polyaromatic hydrocarbon (PAH) ions under static SIMS analysis conditions. The kinetic energy distribution (KED) of most of the secondary ions sputtered from dibenzanthracene exhibits up to three peaks. The peaks corresponding to a negative apparent energy are due to the metastable decomposition of larger ions in the acceleration section of the spectrometer. Their origin, i.e. delayed H and H-2 loss reactions, is discussed in detail. The kinetic energy distributions of the dibenzanthracene secondary ions are compared with those of the PAH ions emitted from polystyrene, especially in the region m/z = 100-200 of the PS mass spectrum. It turns out that similar interpretations can be proposed for polystyrene. To confirm the hydrogen loss reaction in the case of polystyrene, the comparison is also made with the KED of similar but deuterated PAH ions sputtered from deuterated PS. On the basis of these hydrogen loss reactions, the formation times of the daughter ions are calculated. (C) 1997 Elsevier Science B.V.