Cluster ion SIMS

3 Cluster Ion SIMS Dynamic SIMS in its traditional form typically does not provide molecular information. Recent studies have, however, revealed that using large cluster primary ions, molecular depth profiles and even images in all three dimensions can be collected. 

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Electron Impact Sources El sources are heavily used in SIMS to provide reliable beams of any inert gas (Ar is most common) as well as other gases such as O2 (02 ions), SFg (for SEj" " ions), Cgg (for Cgg ions), Cg4 (for C84" ions), Corenene (for C24H12 ), and Argon (Ar where n can extend to several thousand). Multiply charged ions are also produced, and indeed, Cgo and Cgo ions, for example, can be useful when higher impact energies/smaller probe diameters are required. El sources are used in all forms of SIMS, i.e. Static, Dynamic, and Cluster ion SIMS (see Section 4.1.1 and sections within). 

The predominant species observed in SIMS spectra are singly charged atomic and molecular ions [51], However, inorganic and organic cluster ions can also be formed. If the sample consists of a simple single-component metal, then clusters such as M, M, etc., are observed in addition to M+ [52], Oxidation of the metal results in formation of MO ", MO/, M Oll", etc. The relative yield of MO+ to M+ depends on the bond dissociation energy of the oxide [52], For a two-component, oxidized metal, clusters of the type M/", M N, MjO, and M N O/ are observed [51]. 

Secondary ion mass spectrometry (SIMS) is by far the most sensitive surface technique, but also the most difficult to quantify. When a surface is exposed to a beam of ions (Ar", 0.5-5 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection (desorption) of atoms, ions, and multi-atomic clusters. In SIMS, positive or negative secondary ions are detected directly with a quadrupole mass spectrometer. 

We have also carried out preliminary experiments in which we have detected the laser desorption of ethylene, cyanogen, methanol, and benzene from the Pt(s)[7(111) x (100)] surface. These spectra are shown in Figure 9. In the experiments involving ethylene, cyanogen, and methanol only neutral species are desorbed. In the case of benzene we observe the molecular parent ion in the absence of the electron beam. We believe that this is due to resonance multiphoton ionization of the benzene by the laser after desorption (resonance multiphoton ionization of benzene is very efficient with 249 nm radiation). These spectra are in marked contrast to the results of SIMS experiments which produce a wide variety of complex metal-adsorbate cluster ions. In the case of ethylene, our experiments were performed at 140 K, and under these conditions ethylene is known to be a molecular x-bonded species on the surface. In SIMS under these conditions the predominant species is CH (15)t but in the laser desorption FTMS experiments neutral ethylene is the principal species detected at low laser power. 

SIMS Cluster Ion Characterization During Oxygen Adsorption and Oxidation. For heavy oxidation, that is essentially bulk oxide films, the oxidation state of the metal can be determined from the positive and negative SIMS intensity distributions (1 ). Though similar attempts have been made to characterize the nature of the surface during the early stages of oxygen interactions (14,15), we now know from the extensive information available from other techniques that such interpretations are incorrect. We use the by now well-characterized W(100)/O and Ni(100)/0 systems as examples. 

SIMS intensities from the "clean" Cu/Ni surfaces cannot be used to determine Cu/Ni surface concentrations, or relative change in concentration from one surface to another. This is because trace impurities (of very low but unknown concentration) preferentially bond to Ni sites and therefore the Ni containing SIMS cluster ions are preferentially enhanced, leading to an erroneously high determination of Ni concentration. 

Matrix effects and inhomogeneous sample charging seriously hinder quantitative analysis of SIMS on technical catalysts. Although full quantitation is almost impossible in this area, the interpretation of SIMS data on a more qualitative base nevertheless offers unique possibilities. Molecular cluster ions may be particularly informative about compounds present in a catalyst. 

SIMS spectra of the RhCf salt in Fig. 9.1 show clear molecular peaks characteristic of rhodium coordinated by chlorine. In particular the RhCl2 signal is very intense. As explained in Chapter 4, there is little doubt that molecular cluster ions from compounds other than alloys are the result of a direct emission process. Hence, Fig. 9.1 implies that if a sample contains rhodium atoms with more than one chlorine ligand, SIMS is capable of detecting this combination with high sensitivity. 

PHI TRIFT IV ToF-SIMS (Physical Electronics, USA) employs three electrostatic analyzers in the ion path to filter the background and metastable secondary ions. Using liquid metal cluster ion guns (such as Aut ion beam for sputtering of sample surface) increased sensitivity compared to a Ga+ primary ion beam are obtained (www.phi.com). The application of dual primary ion guns is useful for an effective dual beam depth profiling on multi-layered samples. 

Investigations of cluster formation serve to explain the evaporation and atomization of sample material and ion formation processes. A further aim of cluster research is to find out under what conditions cluster or polyatomic ion formation can be influenced in order to avoid disturbing interferences and decrease the detection limits of elements. On the other hand, polyatomic ions have also been used as analyte ions for analysis, e.g. the application of MCs+ and MCs2+ dimeric and trimeric ions as analyte11 or of cluster primary ion beams (e.g., of bismuth and gold primary clusters)15 16 by the bombardment and sputtering of a solid surface in SIMS.17-21 Especially in SIMS, a multitude of cluster ions with high ion formation rates are observed.18 22 23... 

Molecular cluster ions are very useful because they reveal which elements are in contact in the sample. Of course, this presupposes that such clusters are emitted intact and are not the result of recombination processes above the surface. Oechs-ner [13] collected evidence that direct cluster emission processes predominate in the case when relatively strong bonds exist between neighbor atoms. Direct emission becomes even more likely if the two atoms differ significantly in mass, and when the heavier atom receives momentum from the sputter cascade in the solid. Thus, there is little doubt that clusters of the type ZrO+, FeCl3-, MoS+, CH3+, PdCO+, or Rh2NO+ (which we will encounter in the applications later) stem from direct emission processes and reflect bonds present in the sample [2, 4], Some evidence exists, however, that atomic recombination may play a role in the SIMS of metals, and in alloys where the two constituents have comparable mass... 

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