Investigation of the formation process of MCs+-molecular ions during sputtering

In secondary ion mass spectrometry, the detection of MCs+ clusters (with M an element of the specimen) under a Cs+ bombardment is frequently used for the quantification of major elements. Despite some very good results obtain ed by this method, some problems still remain. In order to gain some more i nsight into these problems, the formation mechanism of the MCs+ clusters is investigated using a Monte Carlo model. It is shown that the majority of t he constituent particles of the formed clusters are initially first or seco nd neighbor atoms at the surface and that the velocity distribution of the MCs+ clusters becomes broader and peaked at higher velocities with increasi ng surface binding energy of the M atom. In addition, it is demonstrated th at the interaction potential between the M and Cs+ particle has no influenc e on the velocity distribution of the MCs+ clusters. On the other hand, the cluster formation probability, defined as the probability that a sputtered M and Cs' particle will form a MCs+ cluster, is extremely sensitive to thi s interaction potential. It is also shown that the cluster formation probab ility decreases with increasing surface binding energy. Finally, a good cor respondence is obtained between the calculated and experimental velocity di stributions of MCs+ clusters sputtered from different monoatomic materials. As a consequence, the Monte Carlo model and the discussed results can be v alidated. IT Am Soc Mass Spectrom 2000, 11, 650-658) (C) 2000 American Soci ety for Mass Spectrometry.