Abstract
An experimental investigation of M 2X + cluster ion formation by alkali halides in secondary ion mass spectrometry is presented as evidence of a gas phase cluster ion formation mechanism. Experimental M 2X + ion yields are compared with calculated heats of formation for M 2X + ions formed from binary mixtures of alkali halides, MX (M = Li, Na; X = F, Cl). The cluster ion intensity order was found to be Li 2F + > Li 2Cl + > Na 2F + > Na 2Cl +, consistent with gas phase mechanism predictions. This ordering may be affected, however, by experimental artifacts such as mass discrimination by the quadrupole mass spectrometer, and variations in composition caused by sample preparation or preferential sputtering. Steps are taken to limit quadrupole mass discrimination, and X-ray photoelectron spectroscopy (XPS) is used to examine surface stoichiometry changes induced by sputtering. The XPS study suggests that, although preferential sputtering did occur, it did not influence the relative ordering of cluster ion intensities.
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