On the SIMS Ionization Probability of Organic Molecules

Abstract

The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and post-ionization techniques. The largest factor affecting secondary ion efficiency is believed to be the poor ionization probability (α) of sputtered material, a value rarely measured directly, but estimated to be in some cases as low as 10. Our lab has developed a method for the direct determination of α in a SIMS experiment using laser post-ionization (LPI) to detect neutral molecular species in the sputtered plume for an organic compound. Here, we apply this method to coronene (CH), a polyaromatic hydrocarbon that exhibits strong molecular signal during gas-phase photoionization. A two-dimensional spatial distribution of sputtered neutral molecules is measured and presented. It is shown that the ionization probability of molecular coronene desorbed from a clean film under bombardment with 40 keV C cluster projectiles is of the order of 10, with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. In general, this work establishes a method to estimate the ionization efficiency of molecular species sputtered during a single bombardment event. Graphical Abstract <!-- [INSERT GRAPHICAL ABSTRACT TEXT HERE] -->.

SIMS imaging in neurobiology and cell biology

“…First, the properties of the primary ion beam determine the mass range of the detected molecules. 5 A higher energy of the primary ions produces higher secondary ion yield which increases sensitivity; however it causes higher fragmentation, meaning that the molecular information on the detected ions can be lost. A highly energetic beam is also more destructive to samples.…”

Atom Probe Tomography of Molecular Organic Materials: Sub-Dalton Nanometer-Scale Quantification

“…6,7,15,23,35 Furthermore, because the material is sputtered, it generally creates fragments that make interpreting the resultant mass spectrum more difficult, and can have differences in sputtering efficiency for different materials. 23,37 Even with these limitations, SIMS is closest to a single measurement of chemical and morphological information that is widely used by the community, but still cannot answer many questions about system morphology because of its limited spatial resolution and fragmentation of molecular species.…”

Effect of SIMS ionization probability on depth resolution for organic/inorganic interfaces

“…At this point, we can only speculate about the reason for the changes of the ionization efficiency when going through the interface. In principle, the velocity‐integrated ionization probability measured here can be influenced by variations of the emission velocity distribution . While this dependence has been discussed (and debated) mainly for inorganic materials and ejectees, we feel that it is an unlikely cause for changes in the chemical ionization mechanism responsible for the formation of the quasi‐molecular ions studied here.…”

“…Comparing the mass resolved signal detected for a molecular secondary ion and its neutral counterpart therefore directly delivers information about the SIMS ionization probability . We have used this methodology in the past to determine the formation of molecular secondary ions for a few organic molecules sputtered from a homogeneous film . It was found that even when profiling through such a single component material, the ionization probability changes as a function of applied ion fluence due to the chemical surface modification induced by prolonged ion bombardment.…”