A study of secondary ion correlations using coincidence counting time-of-flight mass spectrometry

Abstract

Coincidence counting techniques have been combined with time-of-flight mass spectrometry (TOF-MS) in a study of secondary ion relationships using keV and MeV projectiles. The method is based on analyzing the results of each primary impact on an event-by-event basis. In this manner, it is possible to monitor the coincidental emission of multiple secondary ions. Two ions observed as a result of the same desorption event are said to be in coincidence with one another. We have demonstrated the capability of coincidence counting TOF-MS to determine the spatial relationship between two sample components, A study of NaF crystals on a PVME substrate shows a decrease in the coincidental emission of secondary ions from the two components as the size of the NaF crystals increased. There is a linear relationship between the size of the NaF crystals and a homogeneity factor, calculated using coincidence counting results. Our data demonstrate that coincidence counting TOF-MS can be used to reveal the homogeneity of a sample surface. A second objective of this research was to examine how coincidence counting results are affected by the characteristics of the primary ions. Specifically, we have adjusted the desorption efficiency of the primary ion beam by varying parameters such as energy, mass, complexity, and charge state of the incident ions. The degree of correlation between specific pairs of secondary ions was observed to be an inverse function of the desorption efficiency of the primary ion. We suggest that the observed degree of secondary ion correlation is primarily a function of the size of the desorption site, which is determined by the characteristics of the primary ion. The correlated emission of secondary ions has also been examined via the number of secondary ions detected per desorption event, i.e, the multiplicity distribution. Using the multiplicity analysis, the emission of multiple secondary ions was found to be more correlated when lower charge state primary ions are used. Additionally, the correlated emission of multiple secondary ions has also been shown to depend on the nature of the sample.