Ion emitters

These thin wires are supported on a special carrier that can be inserted into the ion source of the mass spectrometer after first growing the whiskers in a separate apparatus. Although the wires are very fragile, they last for some time and are easily renewed. They are often referred to as emitter electrodes (ion emitters). 

There are basically two kinds of experiments which can be used for studying the mechanisms of the field ionization process near a field ion emitter surface and also the field ion image formation process. They are the measurement of field ion current as functions of tip voltage, tip temperature, and other experimental parameters, and the measurement of the ion energy distribution. 

The Chevron channel plate ion detector assembly of an imaging atom-probe can also be replaced by a position sensitive particle detector combined with a data processor, as reported by Cerezo etal.5s (A position sensitive detector was used earlier for the purpose of field ion image recording and processing.59) With such a detector both the chemical identity and the spatial origin on the emitter surface can be found for each field evaporated ion. This position sensitive atom-probe can be used to study the spatial distribution of different ion species on the emitter surface as well as inside the bulk of the emitter with a spatial resolution nearly comparable to the FIM. For such a purpose, one carries out the field evaporation at an extremely slow rate so that no more than one ion is detected from the entire field ion emitter surface in each pulsed field evaporation. From the flight time of the ion its chemical species is identified, and from the location of the detector where the ion is detected the spatial origin of the ion is located. With a fast data processor, a two-dimensional distribution of chemical species on the tip surface can be... 

Figure 1. Liquid metal ion emitters (a) capillary, (b) needle.

The vast majority of materials vaporize only neutral species at elevated temperatures, and only a few materials emit ions to an appreciable extent. As an introduction to the topic of ion emitters it is useful to ask the question, What causes a thermally hot condensed phase material to vaporize atoms anckor molecules as ions rather than as neutral species This is a nontrivial question and only recently have some models that address this question begun to emerge for a few ion emitters. These models are still in the conceptual state, and although they are consistent with limited sets of experimental data, ideally they will achieve considerable refinement in the coming years. 

These results illustrate the importance of the chemical species of the element present in the deposit with regard to ion emission (and gives insight into the effect of the oxidizing/reducing nature of the ion emitter) but tell little about the actual mechanisms active in the ion emitting process. As an example, the ions could be emitted either from the deposit itself or from an intermediate material that formed as a consequence of the chemical properties, or it could be entirely an interface phenomenon in which the deposit only served as a repository for the uranium species and the supporting filament served as the ionization surface. 

The study of the mechanistics of ion formation from hot ion emitters has become a topic of research in the author s laboratory in recent years. As with all research, the deeper one probes the more there is to study. Whenever a new level of understanding was achieved, new questions arose that required new approaches and in some instances new instruments custom designed and built to accomplish measurements needed to achieve the next level of understanding. These instruments are described with examples of how they are applied. 

A secondary ion mass spectrometer (SIMS) able to analyze the chemical composition and speciation of the surface of ion emitters at full operating temperature... 

Several new types of source mounts have also been developed to allow optimal placement and operation of the ion emitters in these instruments. These instruments provide complementary information on the properties of ion emitters that are helping to elucidate the mechanisms by which ion emitters function. This knowledge has allowed the development of better ion emitters. 

This instrument [7] measures three types of ions in a sequential mode the positive and negative ions emitted from the surface of the ion emitter, and the neutral species volatilizing from the surface and ionized by electron impact (El). A commercially available quadrupole mass spectrometer equipped with an El source was modified to allot a specially designed thermal emitter to be just barely inserted into the ionization chamber. The chamber is much cooler than the emitter there-... 

The fourth example, an unpublished study [10], has demonstrated that silver molten glass ion emitters volatilize a considerable neutral silver atom flux accompanying the silver ion flux, again leaving open the possibility that this is a pseudo-S-L type of ion emission process. 

Additional evidence, useful in supporting the concept that silver zeolite and silver molten glass ion emitters are S-L processes, would be that both ions and neutrals arise from the same species in the solid state this issue is addressed in the following section, on high-temperature SIMS. 

There are dozens of analytical methods published in both the open literature and in government and other reports that describe the preparation of various ion emitters for the purpose of measuring isotope ratios. There are also many methods that various laboratories have developed that have never been published but are referred to in oral presentations. Thus there are probably many groups of ion emitters that have been developed and used but whose mechanistics properties are not understood. If the mechanism were understood, however, it is likely that the method could be improved. At the beginning of this chapter ion emitters were divided into two categories, presynthesized ion emitters and those with ongoing chemical and physical reactions that produce the ions in situ. Examples of each of these are presented here, and their mechanistic characteristics are described to the extent they are understood. 

It is an experimental observation that only the rare earths starting out in the +3 oxidation state are efficient ion emitters. 

Emitters that have ions produced in situ are by far the largest group of known ion emitters. They are much more difficult to scale in intensity than prefonned ion emitters. In general, methods have been developed that give stable and reproducible ion beams with sufficient intensity to provide an isotope ratio analysis for the particular element to be analyzed. As stated earlier, it is not necessary to understand the mechanistics of ion emission to use these ion emitters for isotope ratio analysis as long as the ion beams are sufficiently stable with adequate intensity. There... 

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