Ion-Bombardment Techniques

The most common mode of chemical analysis presented herein has been the monitoring of elastically/inelastically scattered or recoiled incident beam species, or the analysis of a secondary emission pattern. In addition to the release of characteristic X-rays, Auger electrons, and photoelectrons, an incident beam may cause ionization of the sample. This technique is known as secondary-ion mass spectrometry (SIMS), which represents the most sensitive surface characterization technique developed to date, with detection limits of atoms cm  

Depending on the desired species to be analyzed, a variety of primary ion beams may be used such as Cs+, Oj, O, Ar, and Ga. Whereas a cationic primary beam such as Cs is used to ionize electronegative elements (e.g., O, C, N, chalco-gens, halogens, etc.), oxygen atoms are used to ionize electropositive elements (e.g., Li, Mg, Na, B, Al, etc.). Liquid metal ion beams (e.g., Ga, Au/Ge) are used for high-resolution studies, since the beam may be focused to a smaller diameter ( 50 nm) for micron and nanoscale analyses. 

There are two varieties of SIMS - static and dynamic. Static SIMS (often referred to as time-of-flight SIMS, TOF-SIMS) is often the method-of-choice, used for elemental analysis and imaging of the top two to three monolayers of a sample in comparison, dynamic SIMS is used to determine elemental concentrations of the sample, as a function of depth. As such, dynamic SIMS is a destructive technique primarily used for depth profiling, whereas TOF-SIMS does not appreciably deteriorate the surface being analyzed. For instance, due to a slow, controllable sputtering rate, the entire analysis may be performed without removing less than one-tenths of an atomic monolayer. 

Dynamic SIMS typically uses a quadrupole mass analyzer however, a TOF mass analyzer offers much higher sensitivities (with limitations, vide infra) and mass ranges. In a TOF-SIMS, an ion of known electrical charge, but unknown mass, is accelerated by an electrical field. As a result, all ions of the same charge will have identical kinetic energies. However, the velocity of the ion will depend on the 

Due to more energetic collisions, an incident beams consisting of either heavier ions (e.g., Au vs. Ga ), or polyatomic species (e.g., Aus, SFs, Bis ), 

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For the analysis of surfaces there are a group of ion bombardment techniques based on sputtering processes described in Ref 30 Since the spectra obtained consists mainly of adsorbed gases and radicals, these techniques are omitted from this discussion... 

Chemical vapor deposition, nanowires, 179 Cluster structures change in grain size, 5 ion-bombardment technique, 7-8 Cobalt molybdates, hydrodesulfurization catalysts, 2-3... 

The most-used spectroscopic method in drug analysis is mass spectrometry (MS). It allows (depending on the ionization technique) measurement of impurities at the nanogram or picogram level if an ion bombarding technique is used, even... 

Heating of the specimens can be accomplished by direct resistance heating or by electron bombardment. The latter technique is often used in conjunction with ion bombardment techniques (see Section 2.1.1e). Resistance heating is normally used for metals, but electron bombardment is more useful for semiconductors and for samples which cannot be readily attached to cinrent leads because of the experimental set-up ... 

In order to determine the electronic work function the metal must be ultrapure with no contamination on the surface. The biggest problem in this regard is the fact that most metals have a surface oxide layer under ambient conditions. A clean metallic surface can be formed by sputtering the metal in a vacuum chamber, and techniques are available for growing thin metal films of known crystallographic orientation. Otherwise, the metal surface may be cleaned by positive ion bombardment techniques at very low pressures. Values of the electronic work function for polycrystalline metal surfaces are summarized in table 8.2. The lowest... 

N.J. Mikkelsen and G. Sorensen. Modifications of Molybdenum-Disulphide Films by Ion Bombardment Techniques. In L.E. Pope, L.L. Fehrenbacher, and W.O. Winer, editors. New Materials Approaches to Tribology Theory and Application. Materials Research Society Symposium Proceedings, Volume 140, Materials Research Society, Pittsburgh, 1989, p. 271. 

Atom/ion bombardment techniques are relatively new in organic MS, and the understanding of the effects of various experimental parameters is actively being refined. This chapter will serve to outline the current understanding of the technique and provide a basis for the logical application of derivatives in FAB mass spectrometry studies. 

To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

Charging bv ion bombardment is the technique used in most mineral separations. The conductive-induction (nonionizing) plate types of separators have also been used Application.s of this device in the minerals indiisti v include its use as a final cleaning step when concentrating rutile and zircon,... 

The SNMS instrumentation that has been most extensively applied and evaluated has been of the electron-gas type, combining ion bombardment by a separate ion beam and by direct plasma-ion bombardment, coupled with postionization by a low-pressure RF plasma. The direct bombardment electron-gas SNMS (or SNMSd) adds a distinctly different capability to the arsenal of thin-film analytical techniques, providing not only matrbe-independent quantitation, but also the excellent depth resolution available from low-energy sputterii. It is from the application of SNMSd that most of the illustrations below are selected. Little is lost in this restriction, since applications of SNMS using the separate bombardment option have been very limited to date. 

Because of the inherently destructive nature of ion bombardment, the use of SSIMS alone in the study of the reactions of surfaces with gases and vapor must be viewed with caution, but in combination with other surface techniques it can provide valuable additional information. The parallel techniques are most often XPS,TDS, and LEED, and the complementary information required from SSIMS normally refers to the nature of molecules on surfaces and with which other atoms, if any, they are combined. 

The anion distribution in anodic oxides is usually determined by ion bombardment or chemical sectioning of alumina samples with subsequent analysis by AES or XPS methods, or by the use of the depth-resolving techniques, such as RBS.150 Different types of concentration profiles are shown in Fig. 27. 

Spectra for a series of Cu-Ni alloys have been obtained (91) and these are reproduced in Fig. 11. Because of overlapping of peaks from the component metals, separate indications of each element are only obtained from the 925 eV Cu peak and the 718 eV Ni peak. The results have only qualitative significance because the quoted nickel concentrations are bulk values. Nevertheless, they do suggest that for these particular samples of Cu-Ni alloys, the surface composition varies smoothly from pure copper to pure nickel. Auger spectroscopy has subsequently shown that the surface composition of the (110) face of a 55% Cu-Ni crystal was identical with the bulk composition (95a). Ono et al. (95b) have used the technique to study cleaning procedures argon ion bombardment caused nickel enrichment of... 

The carbinolamine metabolite 64 was unstable in organic solvents and on routine chromatography. It was impossible to obtain molecular ions in the mass spectra by electron impact, chemical ionization, Held desorption, or fast-atom bombardment techniques. The apparent molecular ion occurred at m/e 456, consistent with a carbinolamine such as 64, which eliminates water to form an [M — 18] peak. The H-NMR spectrum of this metabolite was similar to that of... 

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