Total electron yield detection

In concentrated systems obtained in a thin uniform shape, the simplest way to record X-ray absorption data is the transmission mode in which the incident and transmitted photons are directly measured by means of ionisation chambers. However, in dilute systems or for surface characterisations, data are usually recorded using secondary effects resulting from the creation of the core hole during the absorption process and from its subsequent relaxation by radiative or non-radiative decays. These processes are the X-ray fluorescence emission and the total electron yield (TEY) emission, respectively. In these detection modes, the linear absorption coefficient is proportional to the ratio of the fluorescence or TEY intensity to... 

XANES spectroscopy has been used to study the composition and mechanism of antiwear tribofilm formation. The absorption XANES spectra were recorded in total electron yield (TEY) versus fluorescence yield (FY) detection to investigate the chemical nature of P, S, Ca, O and Fe on the surface and in the bulk, respectively. The application of XANES surface TEY mode which analyzes the top 5 nm layer, and the FY technique which analyzes the 50 nm layer of the bulk, taken together, give a marvelous opportunity to study nondestructively the antiwear tribofilms. Both techniques can be used under a wide variety of conditions e.g., the formation of tribofilms at different rubbing times, load, concentrations, temperatures and surface roughness (Kasrai et al., 1993 and 1996 Koningsberger and Prins, 1988 Martin et al., 2001 Yin et al., 1997a). 

Electron yield—Auger, partial, or total—can be employed as a means of detection since again these techniques depend on the generation of core holes. Because of the very small mean free paths of electrons, electron yield detection is very well suited for surface EXAFS measurements. However, due to this very same reason, in-situ studies of electrochemical interfaces are precluded. Details of electron yield EXAFS have been discussed by a number of authors. ... 

Electrons ejected after the core ionization can be measured either selectively by their energy as Auger electrons or unselected as the so-called total electron yield. Due to the small free path that electrons have in condensed matter, these electrons stem from a thin layer of the surface of the sample. Under these conditions, XAS becomes a surface-sensitive probe [41] known as SEXAFS (Surface EXAFS) and NEXAFS (Near Edge X-ray Absorption Fine Structure with the same meaning as XANES,but applied exclusively to near-edge spectra detected using surface-sensitive measurements). These methods have become very important... 

Surface EXAFS (SEXAFS) uses Auger or photo-electrons to detect the EXAFS signal. This ensures that this technique has a much higher surface sensitivity than EXAFS acquired using the total electron yield method. SEXAFS requires ultra-high vacuum and the detection instrumentation normally associated with Auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS) techniques. 

The absorption of the Mj y and N,v y transitions are located in the soft X-ray range ( 800-1700 eV) and in the ultraviolet ( —98-205 eV), respectively. In the early period of X-ray spectroscopy, such absorption measurements, in particular those in the ultraviolet, were hampered by the low intensities emitted from conventional X-ray sources, by the difficulty of preparing thin (— 100 A) uncontaminated samples of defined thickness, and by the involved detection systems. Meanwhile most of these experimental problems have been overcome through the availability of intense and tunable synchrotron sources, the concomitant improvements of the detection systems (e.g., total electron yield detectors) and through developments of appropriate ultra high vacuum preparation techniques. Nevertheless, because of possible intrinsic surface effects, the question of whether one measures bulk properties remains a drawb k here. 

Today soft X-ray absorption spectra (M,N) are routinely recorded by the detection of the total electron yield which is proportional to the absorption coefficient (Bothe 1926). The detection of the total electron yield is advantegeous over the detection of the transmitted intensities through thin (100—3000 A) foils or evaporized layers. Since the spectra are recorded directly from the bulk material, investigations of compounds and dilute alloys are feasible. Experimental problems arise from the surface sensitivity of the method and saturation effects, which are not well understood up to now. 

Fig. 4.4 Two methods of recording X-ray absorption spectra transmission and eiectron yield. The transmission technique requires thin foils while the electron yield technique, often called total electron yield (TEY) detection, can be used for conventional samples. The absorbed X-ray intensity is not measured directly in TEY measurements, but rather the photoelectrons that are created by the absorbed X-rays (Reprinted with permission from Stohr [2])...

Alternatively, the total electron yield from the sample due to cascades initiated by the Auger processes, can be detected (Citrin, 1978). The signal is measured with a charmel-tron detector, or simply by measuring the photoionization current from the sample. This method has the peculiarity of probing a few thousand A under the sample surface (due to the limited electron escape depth) and can be useful in studying macroscopically layered structures, e.g. ion-implanted materials. 

Electron yield—Auger, partial, or total—can also be employed as detection means since again it depends on the generation of core... 

It is true that in some few cases appreciable differences can be detected in both the total -electron charges and the bond orders, reflecting the importance of the choice of parameters. In general it can be observed, however, that most of the values collected are strikingly similar the various methods of integral evaluation do not seem to influence decisively the results. It may be concluded that a Hiickel treatment, carried out carefully, may yield results as satisfactory as a more complicated calculation. On the other hand one could just as well arrive at the conclusion that all these treatments are equally meaningless. 

In the case of AEAPS, it is not necessary to detect individual Auger peaks, as the total secondary electron yield can be measured instead. The electron cascade within the material can act as an electron multiplier increasing the AEAPS signal. Hence an REA could be used or an electron detector of a type used in a scanning electron microscope. 

Even though the vacuum-oriented surface techniques yield much useful information about the chemistry of a surface, their use is not totally without problems. Hydrated surfaces, for example, are susceptible to dehydration due to the vacuum and localized sample heating induced by x-ray and electron beams. Still, successful studies have been conducted on aquated inorganic salts (3), water on metals (3), and hydrated iron oxide minerals (4). Even aqueous solutions themselves have been studied by x-ray photoelectron spectroscopy (j>). The reader should also remember that even dry samples can sometimes undergo deterioration under the proper circumstances. In most cases, however, alterations in the sample surface can be detected by monitoring the spectra as a function of time of x-ray or electron beam exposure and by a careful, visual inspection of the sample. 

In principle, the neutral desorbed products of dissociation can be detected and mass analyzed, if ionized prior to their introduction into the mass spectrometer. However, such experiments are difficult due to low ejfective ionization efficiencies for desorbed neutrals. Nevertheless, a number of systems have been studied in the groups of Wurm et al. [45], Kimmel et al. [46,47], and Harries et al. [48], for example. In our laboratory, studies of neutral particle desorption have been concentrated on self-assembled monolayer targets at room temperature [27,28]. Under certain circumstances, neutrals desorbed in electronically excited metastable states of sufficient energy can be detected by their de-excitation at the surface of a large-area microchannel plate/detector assembly [49]. Separation of the BSD signal of metastables from UV luminescence can be effected by time of flight analysis [49] however, when the photon signal is small relative to the metastable yield, such discrimination is unnecessary and only the total yield of neutral particles (NP) needs to be measured. 

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