Surface photon spectroscopic methods

Surface Charaeferization by Spectroscopy and Microscopy 589 21A Introduction to the Study of Surfaces 589 21B Spectroscopic Surface Methods 590 21C Electron Spectroscopy 591 2 ID Ion Spectroscopic Techniques 602 21E Surface Photon Spectroscopic Methods 604 21F Electron-Stimulated Microanalysis Methods 607... 

In this section we discuss methods in which otons provide both the primary beam and the detected beam. The techniques discussed are listed in Table 21-1 namely, surface plasmon resonance, sum frequency generation, and ellipsometry. The electron and ion spectroscopic surface techniques described previously all suffer from one disadvantage they require an ultra-high vacuum environment and provide no access to buried interfaces. The photon spectroscopic methods described here can all deal with surfaces in contact with liquids and, in some cases, surfaces that are buried under transparent layers. 

The use of optical methods which probe interface electronic and vibrational resonances offers significant advantages over conventional surface spectroscopic methods in which, e.g. beams of charged particles are used as a probe, or charged particles emitted from the surface/interface after photon absorption are detected. Recently, three-wave mixing techniques such as second-harmonic generation (SHG) have become important tools to study reaction processes at interfaces. SHG is potentially surface-sensitive at nondestructive power densities, and its application is not restricted to ultrahigh vacuum (UHV) conditions.However, SHG suffers from a serious drawback, namely from its lack of molecular selectivity. As a consequence, SHG cannot be used for the identification of unknown surface-species. 

This is a third-order nonlinear spectroscopic method that does not involve time delay. It consists of sending two coherent beams on a sample simultaneously one in the visible-UV region and the other one in the IR region and observing the photon that is emitted at the sum of their frequencies and is concomitant with the absorption of two photons, one in each of the two incident beams (72). The spectroscopic regions of the two incident beams are regions of transparency of the sample. The emitted photon requires absence of a centre of symmetry at molecular level to appear. It means that it practically does not appear in the bulk of a liquid, for instance, which is isotrope and consequently displays a centre of inversion in the average, but may appear on its surface, or at the interface between this liquid and another medium, where this centre of inversion disappears. It will consequently be most useful in the study of surfaces and interfaces, particularly the structures of the molecules thereon that can be deduced from the spectrum of these surfaces or interfaces (73). In many situations, it may be the unique tool to study liquid surfaces and interfaces and we shall see this in Ch. 9, which is devoted to liquid water-related examples. 

Figure 21-1 illustrates the general way spectroscopic examinations of surfaces are performed. Here, the solid sample is irradiated with a primary beam made up of photons, electrons, ions, or neutral molecules. Impact of this beam on a surface results in formation of a secondary beam also consisting of photons, electrons. molecules, or ions from the solid surface. The secondary beam is detected by the spectrometer. Note that the type of particle making up the primary beam is not necessarily the same as that making up the secondary beam. The secondary beam, which results from scattering, sputtering, or emission, is then studied by a variety of spectroscopic methods. 

What are the main advantages of surface photon techniques when compared with electron and ion spectroscopic methods What are the major disadvantages ... 

In this chapter an attempt will be made to present the fundamentals of some of the spectroscopic methods used for the characterization of surfaces. As shown the spectroscopic methods represent a very important part of the arsenal used for characterization of surfaces, but in many cases it is difficult to separate the spectroscopic principals of a particular method from those of closely related techniques. Also usage of the term spectroscopy is sometime confusing. A broader definition of spectroscopy wiU be accepted here embracing not only the interaction of matter with electromagnetic radiation/photons, but also the interaction with particles (electrons, ions, molecules). Diffraction, desorption, microscopic, and imaging methods have been excluded from the discussion, but we are not quite rigorous in some cases where these methods are very closely connected to related spectroscopic methods. 

This chapter is divided into several major parts. After an introduction to surface methods in Section 21B. we then discuss electron spectroscopic techniques, ion spectroscopic techniques, and photon spectroscopic techniques to identify the chemical species making up... 

Surface analytical techniques can be classified in terms of the excitating and emitted probe cfr. Table 4.4). The penetration of the physical probe increases fl om ions (ISS, RBS, SIMS) to electrons (XPS) and finally photons (UV/VIS, IR, XRF, etc.). Amongst the photon beam techniques which show some degree of surface sensitivity, in practice only XPS, total reflection X-ray fluorescence (TXRF) and laser-induced mass spectroscopic methods (LMMS),... 

If a surface, typically a metal surface, is irradiated with a probe beam of photons, electrons, or ions (usually positive ions), one generally finds that photons, electrons, and ions are produced in various combinations. A particular method consists of using a particular type of probe beam and detecting a particular type of produced species. The method becomes a spectroscopic one if the intensity or efficiency of the phenomenon is studied as a function of the energy of the produced species at constant probe beam energy, or vice versa. Quite a few combinations are possible, as is evident from the listing in Table VIII-1, and only a few are considered here. 

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