Cluster secondary ion mass spectrometry: an insight into ‘‘super-efficient’’ collision cascades

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 Aun mþ projectiles (1 n 4; m ¼ 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 Au4 impacts than from equal velocity Auþ projectiles. # 2004 Elsevier B.V. All rights reserved