Ionization Probability in Molecular Secondary Ion Mass Spectrometry: Protonation Efficiency of Sputtered Guanine Molecules Studied by Laser Postionization

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.

Molecular SIMS Ionization Probability Studied with Laser Postionization: Influence of the Projectile Cluster

“…We attribute both findings to the chemical damage induced by the C 60 impact, which acts to decrease molecular sputter yields on one hand and on the other hand may liberate free radicals that help the protonation process . In any case, the depth profile data presented here and in refs and indicate that irradiation of the sample surface with a projectile ion fluence of several 10 14 ions/cm 2 is sufficient to establish stable conditions regarding the molecular SIMS ionization efficiency, and therefore the experiments discussed in the remainder of this paper were performed after prebombarding the investigated area with the GCIB operated in dc mode and rastered over a surface area of 100 × 100 μm 2 up to this fluence. The following spectrum acquisition was then performed with the pulsed C 60 + or Ar n + ion beam operated in spot mode and directed to the center of the preirradiated area.…”

ToF-SIMS Depth Profiling of Metal, Metal Oxide, and Alloy Multilayers in Atmospheres of H2, C2H2, CO, and O2

“… 53 Laser-SNMS can be used in depth profiling, 55 thereby improving the ionization yield. 56 , 57 With this method, we can selectively ionize only some of the neutral particles (resonant laser-SNMS) 58 or nonselectively ionize all of them (nonresonant). 52 In the case of resonant laser-SNMS, we must know exactly what is being analyzed, and in the case of the nonresonant approach, we must make sure that the laser intensity is high enough to ionize all the particles.…”

Time‐of‐flight secondary ion mass spectrometry and X‐ray photoelectron spectroscopy study of 2‐phenylimidazole on brass

“…[5][6][7][8][9] Lately, with the development of the gas cluster ion beam (GCIB) sputtering source, new insights into the structure of the surface layer are possible. 10,11 The latter is especially needed to study organics/metal interfaces, as in the case of organic corrosion inhibitors the molecule adsorbs on the metallic surface and initially forms a very thin surface layer. The GCIB is complementary to high-resolution (HR) angle-resolved XPS (ARXPS), but in some cases has a significant advantage, as will also be shown in this work.…”

“…Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) 1–4 and X‐ray photoelectron spectroscopy (XPS) are probably the best methods for studying these properties 5–9 . Lately, with the development of the gas cluster ion beam (GCIB) sputtering source, new insights into the structure of the surface layer are possible 10,11 . The latter is especially needed to study organics/metal interfaces, as in the case of organic corrosion inhibitors the molecule adsorbs on the metallic surface and initially forms a very thin surface layer.…”