Secondary Neutral Mass Spectrometry SNMS

In SNMS, sputtered neutrals are post-ionized before they enter the mass spectrometer. In contrast to SIMS, SNMS does not suffer from the matrix effects associated with the ionization probability of sputtered particles. Here, the sensitivity for a cer- 

I is the intensity of secondary neutrals (expressed as a rate in counts per second) 

Y is the sputter yield the number of atoms ejected per incident ion R is the probability of forming a positive or negative ion csurt is the fractional concentration of the element in the surface layer 

As extensive tables of reliable sputter yields are available, SNMS is much more suitable than SIMS for quantitative work. Interestingly, ionic solids also give significant yields of secondary neutrals, underlining the fact that neutralization processes at the surface are also important for non-metallic samples. 

An alternative for the low detection efficiencies of the emitted particles is to ionize them with a UV laser beam, either in a resonant or non-resonant way [37]. In this way the ionization efficiency increases about a thousandfold and the attractive prospect of doing SNMS under static conditions at sensitivities comparable to those of 

In SNMS, sputtered neutrals are post-ionized before they enter the mass spectrometer. In contrast to SIMS, SNMS does not suffer from the matrix effects associated with the ionization probability of sputtered particles, as here the sensitivity for a certain element is mainly determined by its sputter yield. As sputtering is relatively well understood, excellent quantitation of SNMS has been demonstrated. Moreover, SNMS as a technique was developed much later than SIMS, and has not yet been fully exploited for catalysts. 

The data listed in Table 4.1 show that sputtered particles from clean metal surfaces are predominantly neutral. As we have seen in Section 4.2, sputter yields fall generally in the range of 1 to 10, whereas the yields of secondary ions - the product of Y and R - are almost all below 0.01 for metallic samples hence, typically 99% of the sputtered particles from metals are neutral. The major advantage of SNMS is that the signal intensity of secondary neutrals is [compare Eq. (4-1)]  

Y is the sputter yield, the number of atoms ejected per incident ion  

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Electron-impact (El) Secondary Neutral Mass Spectrometry (SNMS)... 

H. Oechsner. Secondary Neutral Mass Spectrometry (SNMS) Recent Methodical Progress and Applications to Fundamental Studies in Particle/Surface Interaction. Int. J. Mass Spectrom. Ion Proc., 143(1995) 271-282. 

Secondary ion mass spectrometry (SIMS) Secondary neutral mass spectrometry (SNMS) Ion-scattering spectroscopy (ISS)... 

The closely allied topics of secondary neutral mass spectrometry (SNMS), fast atom bombardment (FAB), and laser ablation SIMS are important, but are beyond the scope of this chapter. SNMS is a technique in which neutral atoms or molecules, sputtered by an ion beam, are ionized in an effort to improve sensitivity and to decouple ion formation from matrix chemical properties, making quantification easier. This ionization is commonly effected by electron beams or lasers. FAB uses a neutral atom beam to create ions on the surface. It is often useful for insulator analysis. Laser ablation creates ions in either resonant or nonresonant modes and can be quite sensitive and complex. 

Depth profile investigations by Secondary Neutral Mass Spectrometry (SNMS) of SiBNaC powder, oxidized at 1520 °C in air, together with a SEM image of the intersection of a SiBNaC fiber also exposed to air (50 h at 1500 °C) clearly show the presence of that double layer of approximately 1.2-1.4 pm thickness (Fig. 15). 

Oechsner H (1984) Secondary neutral mass spectrometry (SNMS) and its application to depth profile and interface analysis. In Oechsner H (ed.) Thin Film and Depth Profile Analysis. Topics in Current Physics., vol. 37, pp. 63-85. Berlin Springer. 

In most designs secondary neutral mass spectrometry (SNMS) instruments are to a large extent identical with SIMS instruments so there are usually hybrids that work... 

Sputtering by kilovolt ion beams produces far more neutral species than ions. When static methods are employed, one can assume that a large percentage of these sputtered neutrals will be intact molecules. Thus, there is the very real possibility for improving detection limits (sensitivity) if these neutrals can be observed by post-ionization in the gas phase. Such techniques are known as secondary neutral mass spectrometry (SNMS). ... 

A. 10.3.3 SNMS and RIMS Secondary Neutral Mass Spectrometry (SNMS), also referred to as Sputtered Neutral Mass Spectrometry, is a destructive technique primarily used for examining elemental constituents within solid samples. This technique is closely related to Dynamic SIMS in that an ion beam is used to sputter the solid of interest. The difference lies in the fact that the sputtered neutral population, once ionized, is passed through a mass spectrometer. Ionization is induced via the action of a laser, an electron beam, or plasma (ionization yields vary from 10% for lasers to 1% for plasmas). As the greatest fraction of the sputtered population departs in the neutral state, this methodology provides the advantage of improved detection limits and reduced matrix effects relative to SIMS. Depth resolution can extend to 1 nm. Spatial imaging is generally not carried out. No prior sample preparation is needed, but HV or better conditions are required. 

Development of the techniques Secondary Ion Mass Spectrometry (SIMS) and Secondary Neutral Mass Spectrometry (SNMS) during the last three decades ranks as one of the most important advances in Chemical Physics and Surface Science. 

In 1910 Thomson [ 1J discovered that metals emitted secondary particles when bombarded by his so-called Kanalstrahlen. A small fraction of these "sputtered" particles was found to be charged. Nowadays, the twin techniques of secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry (SNMS), both of which are based on Thomson s discovery, belong among the most powerful surface analytical techniques for the compositional characterization of surfaces. In both the surface to be analyzed is bombarded by ions of several keV energy, the so-called primary ions (PI). Depending on whether the masses of the Pl-induced secondary ions or neutrals are analyzed the terms SIMS or SNMS are used. They can be included under the heading surface mass spectrometry since it must be emphasized here that it is a surface and not, e.g.. the gas phase, that is the target for the mass analytical characterization [2]. 

Secondary ion mass spectrometry (SIMS), secondary neutral mass spectrometry (SNMS), Auger electron spectroscopy (AES) and x-ray photoelectron spectro.scopy (XPS) can provide depth-.selective elemental profiles, but the... 

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