Radiation effects in static SIMS of polymers

The formation of dimerized polystyrene molecules during static SIMS measurements, already reported in a previous paper [Surf. Interface Anal. 20, 549 (1993)] has been further investigated. Samples of styrene oligomers with very narrow molecular weight distribution were prepared by fractionating polystyrene standards by means of supercritical fluid chromatography and then irradiated in situ in a time-of-flight SIMS spectrometer. Pre-irradiated samples have also been studied for comparison purposes. The results obtained confirm that crosslinked (dimerized) species are formed and detected in the very low fluence range, and strongly support the hypothesis that, at least partly, the observation of this kind of Ion beam modification is not due to ion impact overlaps during the measurement, but to primary modification/desorption events.