Molecular ionization probability in cluster-SIMS

Significance 

Recent advances have seen the application of cluster projectiles in state-of-the-art approaches to molecular secondary ion mass spectrometry in reducing the bombardment induced fragmentation and damage accumulation at the sample surface. Previously published studies have successfully utilized small, rather unspecific fragment ions to identify certain molecular species in the investigated sample system. Consequently, there is need to enhance molecular specificity by detecting (quasi-) molecular ions that represent either the intact parent molecule itself or at least larger molecule specific fragments.Nonetheless, the ionization efficiency of such species in the course of the emission event has generally been assumed to be rather low, with values down to the order of 10-5 being occasionally quoted in available literature. Worse off, the procedure of obtaining these estimated values is yet to be demystified. Therefore, the actual value of molecular ionization efficiencies must still be considered to be practically unknown.

Recently, Professor Andreas Wucher from the Faculty of Physics at University of Duisburg-Essen carried out experiments that would provide the missing hard evidence regarding the generally assumed ionization efficiency of molecular species sputtered under bombardment with cluster ion beams. He reported on recent combined molecular secondary ion and neutral mass spectrometry (SIMS/SNMS) experiments utilizing laser post-ionization in order to determine a reliable estimate of an absolute ionization probability value for a few organic molecules sputtered under C60 cluster ion bombardment. His work is currently published in the research journal, Journal of Vacuum Science & Technology B.

In brief, the research method employed commenced with the definition of the ionization probability (or efficiency) of a sputtered species by comparing mass spectrometric signals of desorbed molecular secondary ions and the respective neutral molecules. Experimentally, time-of-flight (ToF) mass spectrometry was used in combination with two different laser post-ionization strategies to detect the sputtered neutral molecules.The results are discussed in terms of various secondary ion formation mechanisms.

The author mainly observed that the measurements performed for coronene or guanine molecules revealed that the currently achieved intrinsic ionization efficiency obtained under bombardment with C60 or Arn cluster ion beams was of the order of several 10-3, thereby leaving a headroom of about 2 orders of magnitude for improvement of the detection sensitivity of (quasi-)molecular secondary ions via enhancement of the ionization efficiency.Moreover, by analyzing the measured molecular laser post-ionization signal, the author was able to demonstrate that about the same headroom existed for improvement of the currently achieved post-ionization efficiency.

In summary, the study by Professor Andreas Wucher presented a detailed examination of the notion regarding the relatively low molecular useful secondary ion yield, which is generally observed in cluster-SIMS experiments and often attributed to the poor ionization efficiency of the molecules desorbed from the surface. In his report he established that for chemical ionization, the ionization efficiency could be significantly enhanced via projectile induced surface chemistry if suitable chemically reactive projectile clusters were to be used. Altogether, the results presented may be useful to guide new developments aimed at enhancing the detection sensitivity in molecular cluster-SIMS experiments.

“…maybe it is time to ask ourselves if poor ionization probability alone can be the sole reason for the observed low useful yield in molecular SIMS.”, said Professor Andreas Wucher in a statement to Advances in Engineering.

About the author

Andreas Wucher graduated in 1981 from the Technical University Clausthal in Germany, where he earned his diploma degree in physics. He then moved to the University of Kaiserslautern and obtained his doctorate degree (Dr. rer. nat) in 1986 with fundamental research regarding the ion-induced desorption (“sputtering”) of atoms and molecules from solid surfaces. He afterwards spent six months in an industrial company manufacturing surface analytical equipment, before he moved to the US and spent a post-doc year at the IBM T.J. Watson Research lab in Yorktown Heights, NY, where he worked on methods for mass spectrometric chemical surface analysis using sputtered neutral particles. After his return to Germany in 1988, he established an independent research group at the University of Kaiserslautern, where he obtained his habilitation in 1994. Since 1998, he holds a position as a Professor of Experimental Physics at the University of Duisburg-Essen.

His scientific work concentrates on fundamental research of ion-surface interaction processes with particular emphasis on the sputtering process and its application in different technological areas. One of the main topics that has continued to be in the focus of his research concerns the ionization process as a fundamental basis for the well-known and widely-used Secondary Ion Mass Spectrometry technique.

Reference

Andreas Wucher. Molecular ionization probability in cluster-SIMS. Journal of Vacuum Science & Technology B, volume 36(3), 2018

Go To Journal of Vacuum Science & Technology B

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