Noble Gas Plasmas

The first involves noble gas plasmas and has previously been known as casing . Here, the principal effect is one of surface cross-Unking and eUmination of potential weak boundary layers, but a very usefiil effect is the generation of a barrier layer to additive diffusion. 

Several ion sources are particularly suited for SSIMS. The first produces positive noble gas ions (usually argon) either by electron impact (El) or in a plasma created by a discharge (see Fig. 3.18 in Sect. 3.2.2.). The ions are then extracted from the source region, accelerated to the chosen energy, and focused in an electrostatic ion-optical column. More recently it has been shown that the use of primary polyatomic ions, e. g. SF5, created in FI sources, can enhance the molecular secondary ion yield by several magnitudes [3.4, 3.5]. 

Typical ion sources employ a noble gas (usually Ar). The ionization process works either by electron impact or within a plasma created by a discharge the ions are then extracted from the region in which they are created. The ions are then accelerated and focused with two or more electrostatic lenses. These ion guns are normally operated to produce ions of 0.5-10 keV energy at currents between 1 and 10 pA (or, for a duoplasmatron, up to 20 pA). The chosen spot size varies between 100 pm and 5 mm in diameter. 

Further designs of ion sources applied in plasma spectroscopy such as electrodeless microwave induced plasmas (MIPs) operating in a noble gas atmosphere at low power (mostly below 200 W) or capacitively coupled microwave plasma using Ar, He or N2 the as plasma gas (at 400-800 W) were described in detail by Broekaert.33 Microwave plasmas produced by a magnetron are operated at 1-5 GHz. Their special application fields for selected elements and/or element species are based (due to the low power applied) in atomic emission spectrometry.33... 

Ar . Long-lived metastable states of negative ions of the noble gas atoms other than He are unknown to the present, although Bethge indicated he has extracted very low intensity Xe ion beams from his dense plasma Penning source. Since so little other information is available at this time, it is not inappropriate to adopt He as a model for the isoelectronic monatomic ions of Group VII and for other species. 

In the case where we have to consider the norm temperature for a line of an element which is only present as an impurity in a plasma, e.g., one formed in a noble gas, the dilution in the plasma (a) also has to be considered. For a system with more components P is given by ... 

The plasma is operated in an atmosphere of a noble gas or often also in O2 or C02, so as to create favorable dissociation and excitation conditions or to avoid intense bands such as the CN bands. Universal procedures for the analysis of oxide powders by dc arc analysis are well known and are still in use (see e.g. Ref. [351]). 

Since noble-gas chemistry was initiated in 1962, there has been interest in the possibility of making a xenon-carbon bond. Experiments in 1979 appeared to have provided a solution. When xenon difluoride was bled into the tail of a trifluoromethyl radical plasma, a volatile, waxy white solid was produced and trapped at — 196°C. The reported properties of this material are consistent with the formulation, Xe(CFi3)2 (164), but until this is independently confirmed, some doubt remains about its authenticity. 

Compound films such as oxide, nitride and carbide can be produced by reactive sputtering as mentioned above. Electrically non-conducting substances are processed by rf sputtering. Whether the noble gas ions required for sputtering originate from a gas discharge plasma in the plant, from special ion guns or from other set-ups does not have any influence on the choice of material. 

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