SURFACE AND INTERFACE ANALYSIS, zyxwvutsrqp VOL. 20, zyxwvuts 549-551 (1993) zyxwvuts SHORT COMMUNICATION Dimerization of Polystyrene during Static SIMS Measurements Antonino Licciardello* and Salvatore Pignataro Dipartimento di Scienze Chimiche, Universita di Catania, viale A.Doria 6,I-95125 Catania, Italy Angelika Leute and Alfred Benninghoven Physikalisches Institut der Universitat Munster, Wilhelm-Klemm-Strasse 10, D-4400 Munster, Germany In this work we report on the results of some static time-of-fight secondary ion mass spectrometry (TOF-SIMS) experiments performed on low-molecular-weight narrow distribution polystyrene. Evidence of cross-linking during analysis is found, as indicated by the presence of peaks corresponding to molecules formed by linking of two initial polymer chains. The effect, which can be easily taken into account, does not appreciably affect the molecular weight distribution parameters obtained. However, the observed phenomenon gives the indication that, at least in the case of particularly sensitive systems, ion beam-induced modification can occur also in static SIMS experi- ments. In addition, the observed phenomenon can be of relevance in modelling the sputtering of large molecules. INTRODUCTION Static secondary ion mass spectrometry is a powerful technique in the study of large and labile organic mol- ecules and has been successfully applied in the study of a wide variety of macromolecular systems.’ The crucial feature of the technique is the use of sufficiently low primary ion fluences in order to minimize the effect of ion-beam-induced surface modification of the sample. The maximum fluence compatible with the ‘static’ con- ditions is thought to depend on the nature of the macromolecular system under consideration. For the study of some unsaturated polymers, for example, an upper limit of 3 zyxwvutsrq x 10” ions cm-’ has been proposed.’ More recent papers give the indication that a lower fluence can be even more a p p r ~ p r i a t e . ~ . ~ On the other hand, independent studies’ carried out on narrow distribution polystyrenes show that this polymer suffers an appreciable chemical modification (cross-linking) when irradiated with energetic (100-400 keV) ion beams at fluences as low as zyxwvutsr 2.5 x 10” ions cm-*. This is shown by the appearance of a high-mass tail in the molecular weight distribution determined by gel permeation chromatography. Although the beam energies used in these studies are higher than those typi- cally used in SIMS experiments, the common feature with static SIMS is that overlap of individual ion tracks is thought to be negligible, i.e. the effects observed at low ion fluences are due to isolated ion impact pheno- mena. Estimations based on the results of these high- energy (- 10’ keV) ion irradiation experiments and on the known energy deposition parameters for ion beams typically used in SIMS (10-100 keV) suggested6 that ion impact-induced cross-linking should be significant * Author to whom correspondence should be addressed. 0142-2421/93/060549-03 $06.50 zyxwvutsrqp 0 1993 by John Wiley & Sons, Ltd. also under the experimental conditions of static SIMS. Two possible mechanisms for the detection of such cross-linked species as secondary ions can be hypothe- sized: the same ion impact producing the modification is responsible for the emission (sputtering) of the modi- fied species; a second ion impact is necessary to sputter a species produced by a previous ion impact. The aim of this work is thus to investigate the pres- ence of the above-discussed ion beam effects in static SIMS by means of time-of-flight SIMS (TOF-SIMS). The experiments reported here were performed on low- molecular-weight narrow distribution polystyrene (PS), which is the same material used in the previously men- tioned high-energy irradiation experiments. In addition, the SIMS spectrum of PS is well known and careful studies on the fragmentation mechanisms both in the low-mass range’ (0-200 Da) and in the high-mass range’ (up to 5000 Da) are available in the literature. As to the high-mass region of the spectrum, TOF-SIMS has been proved to be very useful in the study of poly- mers owing to its capability of providing direct informa- tion on the molecular weight distribution (M WD) of different classes of polymers.*-’l In addition, owing to the unparalleled surface sensitivity of the technique, it is possible to obtain routinely the static SIMS spectra even in the high-mass range by using primary ion flu- ences below 10” ions cm-’. EXPERIMENTAL The TOF-SIMS measurements were performed on the combined SIMS-PDMS instrument developed at the University of Miinster, equipped with a reflector-type TOF analyser and an electron impact pulsed ion gun, fully described elsewhere.” Samples of narrow distribution polystyrene zy (Mw = 800, Mw/Mn < 1.3, from Supelco) were prepared in very Received 5 June 1992 Accepted 20 November 1992