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 .

The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and postionization techniques. An important factor affecting the detection sensitivity in molecular SIMS and other desorption techniques as well is believed to be the poor ionization probability of a sputtered molecule, a value which is often assumed to be as low as 10 −5 but at present is basically unknown. In order to estimate how much headroom there is for future developments toward strategies aimed at enhancing the ionization probability, we study the protonation efficiency of sputtered guanine molecules for formation of [M + H] + secondary ions using strong field laser postionization (LPI) to detect the corresponding neutral molecules. In order to allow a quantitative comparison of secondary ion and neutral yields, the postionization signal is corrected for undersampling of the principally detectable plume of sputtered neutral particles by the focused laser beam. It is shown that the protonation probability of molecular guanine desorbed from a clean film under bombardment with 20 keV C 60 cluster projectiles is of the order of 1−2 × 10 −3 , with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. Moreover, we find that the postionization signal can in principle be boosted by 2 orders of magnitude if a more powerful ionization laser is employed.

Strategies to reduce and overcome matrix effects in molecular secondary ion mass spectrometry (SIMS) are investigated using laser-based post-ionization of sputtered neutral organic molecules released under C 60 + bombardment. Using a twocomponent multilayer film similar to that employed in a recent VAMAS interlaboratory study, SIMS depth profiles of the protonated and deprotonated quasi-molecular ions of two well-studied organic molecules, Irganox 1010 and Irganox 1098, were measured along with that of the corresponding neutral precursor molecules. When compared to composition-dependent ionization probability changes of the secondary ions, the resulting profiles are much improved. We demonstrate that detection of neutral molecules via laser post-ionization yields significantly reduced matrix effects when compared to SIMS depth profiles in both positive and negative secondary ion mode. These results suggest that this approach may provide a useful pathway for acquiring depth profiles from complex organic samples with improved capabilities for quantitation.

Strong field ionization (SFI) was applied for the secondary neutral mass spectrometry (SNMS) of patterned rubrene films, mouse brain sections, and Botryococcus braunii algal cell colonies. Molecular ions of rubrene, cholesterol, C31 diene/triene, and three wax monoesters were detected, representing some of the largest organic molecules ever ionized intact by a laser post-ionization experiment. In rubrene, the SFI SNMS molecular ion signal was ~4 times higher than in the corresponding secondary-ion mass spectroscopy (SIMS) analysis. In the biological samples, the achieved signal improvements varied among molecules and sampling locations, with SFI SNMS, in some cases, revealing analytes made completely undetectable by the influence of matrix effects in SIMS.

Secondary ion mass spectrometry (SIMS) relies on the fact that surface particles ejected from a solid surface are ionized under ion bombardment. By comparing the signal of molecular secondary ions desorbed from an organic film with that of the corresponding sputtered neutral precursor molecules, we investigate the variation of the molecular ionization probability when depth profiling through the film to the substrate interface. As a result, we find notable variations of the ionization probability both at the original surface and in the interface region, leading to a strong distortion of the measured SIMS depth profile. The experiments show that the effect can act in two ways, leading either to an apparent broadening or to an artificial sharpening of the observed film-substrate transition. As a consequence, we conclude that care must be taken when assessing interface location, width, or depth resolution from a molecular SIMS depth profile.