Andreas Wucher oral presentation (B&N-Tue1-2-1)
Molecular Ionization Probability in Cluster-SIMS
Fakultät für Physik - University Duisburg-Essen, Lotharstr., 47048 Duisburg, Germany
Current state of the art approaches to molecular Secondary Ion Mass Spectrometry (SIMS) involve the use of cluster projectiles in order to reduce the bombardment induced fragmentation and damage accumulation. In these experiments, one of the key factors limiting sensitivity is the ionization efficiency of sputtered molecular species. Although many published applications have successfully utilized small, rather unspecific fragment ions to identify certain molecular species in the investigated sample system, it is principally desirable to enhance molecular specificity by detecting either the intact parent molecule itself or at least larger molecule specific fragments. The ionization efficiency of such species is generally assumed to be rather low, with values down to the order of 10-5 being sometimes cited in the literature. However, it is rarely reported how these estimates have been obtained, so that the actual value - and even the order of magnitude - of molecular ionization efficiencies must still be considered to be practically unknown. The talk will therefore first report on recent experiments utilizing laser post-ionization (LPI) in order to determine a reliable estimate of an absolute ionization probability value for a few organic molecules sputtered under C60 cluster ion bombardment. The results show that the molecular ionization efficiency obtained under these bombarding conditions is of the order of 10-3, leaving at least two orders of magnitude for sensitivity improvement. Analyzing the measured molecular LPI signal, we demonstrate that about the same headroom exists for improvement of the currently achieved post-ionization efficiency. Then, we will present an overview of different attempts to utilize projectile induced surface chemistry in order to enhance the ionization efficiency of sputtered molecules. This strategy turns out to be particularly important in cases where (rare) gas cluster ion beams (GCIB’s) s are used to generate the measured SIMS signal. For trehalose as a model system, we will illustrate the level of improvement which is currently obtained using reactive gas cluster projectiles. In addition we will try to analyze published useful yield data obtained under different bombarding conditions in order to extract information about the variation of the ionization efficiency as a function of experimental parameters such as nature, size, composition and energy of the projectile clusters. The results may be useful to guide new developments aimed at enhancing the detection sensitivity in molecular cluster-SIMS experiments.