Cluster secondary ion mass spectrometry: an insight into super-efficient" collision cascades
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Some collision cascades, induced by keV polyatomic projectiles, result in the emission of multiple secondary ions. Such coincidental ion emission implies that the ejecta originate from molecules co-located within a nano-volume perturbed by a single projectile impact, thus providing an approach for the chemical analysis of nano-domains. The relevance for the chemical analysis of nano-structures depends on the effectiveness of the projectile to cause co-emission of two or more analytically significant secondary ions. The experiments involved Au nm+ projectiles (1n4; m=1, 2) on phenylalanine targets. We have measured the yields for events in which multiple ions were detected as a function of projectile characteristics. The data show that some collision cascades are "super-efficient". For example, in a four-ion detection event, the yield for the phenylalanine molecular ion is two orders of magnitude larger from Au 4 impacts than from equal velocity Au + projectiles. 2004 Elsevier B.V. All rights reserved.