Secondary ion mass spectrometry (SIMS) of metal halides. IV. The envelopes of secondary cluster ion distributions

Abstract

The relative secondary ion intensities of cluster ions of the type [M(MI) n ] + and [I(MI) n ] − from the fast atom bombardment mass spectrometry (FABMS) of alkali iodides (MI) for n<100 are presented, and mass-resolved mass spectra of cesium iodide (CsI) clusters exceeding m/z 25 000 are reported. The FABMS spectra are qualitatively consistent with previously reported secondary ion mass spectrometry (SIMS) spectra. The envelopes of the extended mass spectra are fitted to the distributions predicted by a model based on random bond breaking. The bond-breaking model (BBM) distributions fit well on the overall envelope of both the positive and negative ion sodium iodide (NaI, simple cubic) and CsI (body-centered cubic) cluster ion distributions, when Bethe lattices of the appropriate coordination number are used to approximate the true lattices. Ion abundance enhancements at certain “magic numbers” in the envelopes can be predicted by the BBM; however, these enhancements and the reduced ion abundance immediately following the “magic number” are consistent with experimentally observed unimolecular, or more appropriately, unicluster decompositions. Finally, we contrast secondary cluster ion distributions with those obtained using other cluster sources and we discuss in detail possible origins of the large cluster ions observed in these studies.