Excitation, surface analysis

Energetic particles interacting can also modify the structure and/or stimulate chemical processes on a surface. Absorbed particles excite electronic and/or vibrational (phonon) states in the near-surface region. Some surface scientists investigate the fiindamental details of particle-surface interactions, while others are concerned about monitormg the changes to the surface induced by such interactions. Because of the importance of these interactions, the physics involved in both surface analysis and surface modification are discussed in this section. 

Analysis of Surface Elemental Composition. A very important class of surface analysis methods derives from the desire to understand what elements reside at the surface or in the near-surface region of a material. The most common techniques used for deterrnination of elemental composition are the electron spectroscopies in which electrons or x-rays are used to stimulate either electron or x-ray emission from the atoms in the surface (or near-surface region) of the sample. These electrons or x-rays are emitted with energies characteristic of the energy levels of the atoms from which they came, and therefore, contain elemental information about the surface. Only the most important electron spectroscopies will be discussed here, although an array of techniques based on either the excitation of surfaces with or the collection of electrons from the surface have been developed for the elucidation of specific information about surfaces and interfaces. 

What do the X-rays do They penetrate down into the solid, through the surface and surface region in which one is interested. On the way, these X-rays cause electrons to be emitted from the atoms or molecules that they meet (the excitation process). Analysis shows that the electrons emitted come not from the outer shells, but from the inner ones. What happens to these electrons It depends on how deep they are in the material. Typically, electrons do not reach the surface if they are emitted from deep inside the electrode. But if the elections belong to atoms closer to the surface, say a few nanometers, they escape into the vacuum... 

Literature analysis and our preliminary experiments with proteins, carbohydrates and other molecules deposited on the surface of nanosilicas suggest that newly developed MALDI MS techniques, in particular Surface MALDI MS, can be adopted as a powerful new tool for surface analysis. Unique capabilities that surpass established techniques in surface analysis of biomaterials using these newly developed methods is an exciting development. 

The whole field received a new impetus after the first oil crisis, when Fujishima and Honda reported on the photoelectrolysis of water at Ti02-electrodes [13], Whereas, before the oil crisis, most basic models and results had been published only by 3-4 research groups in the world, many other scientists entered the field after this crisis and studied solar applications, and hundreds of papers were published. Since then, many processes at semiconductor electrodes have been studied more quantitatively by using not only standard electrochemical methods, but also new techniques, such as spectroscopic surface analysis (see e.g. [12]). Naturally, photoeffects played a dominant role in these investigations. These were not only restricted to reactions induced by light excitation within the semiconductor electrode [11], but were also extended to the excitation of adsorbed dye molecules [14,15]. 

Both electron microprobe analysis and SAM use an electron beam for excitation of the specimen. The difierence between these techniques is in the detection of emitted x-rays in the microprobe technique while SAM measures emitted electrons. For both techniques, the energy of the detected particles is characteristic of the parent atom and thus identifies the atomic species present. The lateral spatial resolution in SAM is superior due to the much shorter mean free path of the emitted energy (electrons). The escape depth of auger electrons is approximately 10 A versus 1000 A in microprobe analysis. This phenomenon makes SAM a highly specific surface analysis technique. 

Figure 7.9 An XPS spectrum of silver excited MgXa with pass energy of 100 eV. (Reproduced with permission from D. Briggs and M.P. Seah, Practical Surface Analysis, Volume 1, John Wiley Sons Ltd, Chichester. 1990 John Wiley Sons Ltd.)...

Engelhardt, H., Duwe, H.P., and Sackmann, E. (1985) Bilayer bending elasticity measured by Fourier analysis of thermally excited surface undulations of flaccid vesicles. Journal of Physics Letters, 46 (8). L395-L400. 

If the sample is a bulk solid sample and is an electrical conductor, it is possible to use it as the cathode of a kind of spectral lamp whose functioning principle is identical to that described for a hollow cathode lamp (cf. Section 13.5.1 and Figure 14.5). The atoms sputtered and removed from the surface of the sample are excited by the plasma. This GD-OES technique provides a rapid and accurate surface analysis, less susceptible to matrix effects and sample homogeneity. It has the advantage of yielding spectra with low background levels whose emission lines are narrow since atomization takes place at lower temperatures than that of the previous techniques. 

In TXRF, involving irradiation of an optically flat sample with a parallel X-ray beam below the angle of total reflection, the depth penetration of the primary X-rays is confined to a few tens of nanometers below the surface. The technique of a-XRF, based on the confinement of the analytical region of the sample, involves the localized excitation and analysis of a microscopically small area of the surface, providing information on the lateral distribution... 

Of numerous interaction processes of photons, electrons and ions with solid surfaces resulting in the emission of excited particles and radiation, or in a characteristic change of the incident species properties, those which can be applied for surface analysis are listed in Table 2. The most frequently used techniques are boxed in the table. 

The basic measuring strategy of microscopic X-ray fluorescence analysis (g-XRF) is illustrated in Fig. 11.23. This microanalytical variant of bulk EDXRF is based on the localized excitation and analysis of a microscopically small area on the surface of a larger sample, providing information on the lateral distribution of major, minor and trace elements in the material under study. Essentially, a beam of primary X-rays with (microscopically) small cross-section irradiates the sample and in-... 

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