Chemistry
Photoelectron Spectroscopy
Photoelectron spectroscopy is a technique used to study the energy levels of electrons in atoms and molecules. By measuring the kinetic energy of photoelectrons emitted when a sample is exposed to light, valuable information about the electronic structure and chemical bonding can be obtained. This method is widely used in chemical analysis and materials science to understand the composition and properties of various substances.
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11 Key excerpts on "Photoelectron Spectroscopy"
- Theodore Kuwana(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
C h e m i s t r y at Surfaces . . . C. Energy Loss Features . . . D. P o l y m e r Surfaces . . . . A p p e n d i x I A p p e n d i x II R e f e r e n c e s I. Introduction A. Historical Perspective The development of electron spectroscopy has recently progressed to the point where it must be considered a necessary tool in the characterization of a wide variety of materials. The technique involves the irradiation of a sample with a monochromatic beam of photons with energy hv. Electrons with a binding energy less than hv are then ejected into the vacuum, where their energy is carefully measured. The phenomenon of photon induced electron emission, i.e., photoemission, has a history dating back to Hertz's observation in 1887 of the initation of sparks across a gap by exposure to ultraviolet light (D'Ageostino, 1975). The explanation of the photoelectric effect by Einstein (1905) and the early experimental studies by Robinson and Rutherford (1913) on measuring ß-ray spectra set the stage for major devel-opments after the second World War. Although a lot of activity took place up to this period, as noted in a recent historical review of the subject (Jenkin et al, 1977), major progress in studying the electronic structure of materials did not occur until the late sixties, primarily due to experimental difficulties. The first application of photoemission to chemical analysis probably dates to Steinhardt (1950; Steinhardt and Surfass, 1953), though his group never had the resources to bring the initial ideas to fruition. During the 1950s and 1960s, however, a major effort was undertaken by Professor Kai Siegbahn and his associates at Uppsala University in Sweden to develop electron spectroscopy as a viable tool in electronic structure determination.
eBook - PDF- Sam Zhang, Lin Li, Ashok Kumar(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
27 3 X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy This chapter covers atomic model, electron configuration, principles, instrumentation, routine limits, and case studies in the application of x-ray Photoelectron Spectroscopy (XPS) and Auger electron spectroscopy (AES). Since XPS and AES have many things in common both in theory and instrumentation—in fact, they are usually in the same laboratory system—these two techniques are discussed in one chapter. 3.1 Introduction XPS is synonymous with electron spectroscopy for chemical analysis (ESCA). XPS is termed to reflect the electron generation process (an x-ray is used to generate photoelectrons that produce “spectroscopy”), whereas ESCA emphasizes the ultimate aim of chemical analysis using electron spectroscopy. From now on, we will only use the term “XPS.” XPS was developed as a surface analysis technique in the mid-1960s by Siegbahn et al. [1] to measure the elemental composition, chemical stoi-chiometry, chemical state, and electronic state of the elements that exist in a material. Shining a torchlight on any surface splashes away some surface electrons because the electrons obtain photo energy and thus are “excited.” XPS spectra are obtained in a similar manner, except that x-ray is used instead of a torchlight because a very-high-energy “photon” is needed to penetrate the material surface and “dig out” the electrons that bond in the inner “core”-level orbital. The electrons generated in this man-ner are called x-ray photoelectrons. As these photoelectrons are captured and their kinetic energies ( E K ) are analyzed and plotted, “Photoelectron Spectroscopy” is created. By analyzing the kinetic energy, the bonding energy of electrons is calculated; thus the material from which the elec-trons escape is identified.
eBook - PDFConjugated Polymer And Molecular Interfaces
Science And Technology For Photonic And Optoelectronic Application
- William R. Salaneck, Kazuhiko Seki, Antoine Kahn, Jean-Jacques Pireaux, William R. Salaneck, Kazuhiko Seki, Antoine Kahn, Jean-Jacques Pireaux(Authors)
- 2001(Publication Date)
- CRC Press(Publisher)
In particular, the method is very useful for studies of the chemical and electronic structure of conjugated polymers and interfaces with polymers because (a) it provides a maximum amount of chemical and electronic in- formation within a single technique, (b) it is essentially nondestructive to most organic systems, and (c) the method is extremely surface sensitive. Although the purpose here is not to give a full understanding of PES, it is worthwhile to discuss some of the specific features in a photoelectron spectrum that may be helpful for the understanding of the different examples discussed in this chapter. The main emphasis in the background to PES will be on the molecular solids aspect because this chapter deals with condensed conjugated systems. The interested reader can find a more in-depth dis- cussion on the technique, relative to organic polymeric systems, in Refs. [11-13]. The fundamental process in molecular photoionization is represented by M 0 + hv^Ml + e (1) where M 0 represents the isolated neutral molecule, hv is the ionizing photon, M* represents the positive molecular ion in the excited state, and e~ is the photoelectron which carries kinetic energy E k . If E k is sufficiently large, then the escaping electron and the molecular ion are not strongly coupled. The energy balance according to Eq. (1) is then given by E 0 + hv = E* + E k (2) where E 0 is the total energy of the neutral molecule in the ground state and E * is the total energy of the positive molecular ion in an excited state. Thus, the basic equation used in interpreting photoelectron spectra can be written as El = E* - E 0 =hv - £ k (3) The photon energy hv is known and the photoelectron kinetic energy dis- tribution is measured in order to deduce the binding energy, E V B .
- F.C. Nachod(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Photoelectron studies using radiation energies in the range between the two classes have not been made primarily because of the lack of any suitable light source operating in this range. However, synchrotron radiation, when used with a monochromator, may become of great use in the intermediate range. This article deals in detail with molecular Photoelectron Spectroscopy and also covers ESCA work where it is necessary to illustrate the comparative strengths and weaknesses of the two techniques and the types of chemical information each provides. Although Photoelectron Spectroscopy is to date the most developed and most useful technique to the chemist, it is only one 1. Photoelectron Spectroscopy 3 member in the general field of electron spectroscopy, a term which may be applied to any method which involves the analysis of electron energies. Other members of this field are mentioned very briefly below, though not discussed further in this chapter, because they give information of a similar nature to that obtained from molecular Photoelectron Spectroscopy and ESCA spectroscopy. AB + (/?th IONIZED STATE) M A ) FIG. 1. Excitation of molecule AB from its ground electronic and vibrational state to an ionized (AB + ) electronic state, at the v = 3 vibrational level. Rota-tional excitations are not shown. The impacting radiation, hv, of Eq. (1) may be replaced by a monoenergetic beam of electrons, and this technique is used to study electronic excitation processes not resulting in ionization and the subsequent ejection of an addi-tional electron. This is done by analyzing the energy loss spectrum of the scattered impacting electrons. The information obtained directly parallels that obtainable from ultraviolet and vacuum ultraviolet absorption spec-troscopy, with the exception that both the spin and orbital selection rules may be different. Electron impact using polychromatic electron beams can also be used to study Auger processes (see Section V, F), often providing
- SachchidaNand Shukla(Author)
- 2019(Publication Date)
- Arcler Press(Publisher)
The emerging technique of this group is the examination of the wide energy loss fine structure (EELFS) spectra, which permits researchers to find the atomic structure on the surface (Kovalev et al., 1999). The second group bonds the approaches in which the release of electrons from the sample surface model is excited by a photon beam. The secondary electrons, in this case, which left the atoms on the surface as a consequence of the complete quantum absorption, are examined (Takats et al., 2004). X-ray Photoelectron Spectroscopy of the inner and outer shells of atoms is founded on this outcome. In spite of the fact that electron release is extensively used for examining surface topographies, which has caused in the additional growth of surface science, all approaches on the basis of this outcome have many stern restrictions. The special surface preparation and ultrahigh vacuum of samples to stop random contamination are essential for researches (Chambers, 2004). Introduction to Modern Instrumentation Methods and Techniques 104 Electron spectroscopy narrates to nondestructive examination approaches . Though, fine variations in the surface state in the development of investigation should always be measured when surface singularities are under examination. The tough treatment of the surface by x-rays or electrons can source the ionization of atoms on the surface, the phenomena of desorption and adsorption, and chemical reactions and the dissociation of molecules on the surface. All these negative surface alterations should be measured when gaining data, numerous of which can be deserted by special experimental methods (Wilson et al., 1980). The achievement of fine-surface studies is founded on the usage of very precise apparatus joint with the services of investigators and their deep knowledge of the physics of the contact of radiation with the substance.
eBook - PDF- Bernd Crasemann(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
In photoionization, however, the measurement of the photo-electrons may serve equally well, offering an alternate, direct approach. The vacancy configuration as well as the details of the process can then be specified and mutually confirmed by both the photoelectron and the Auger electron in a simultaneous, but independent, measurement or in a more sophisticated coincidence arrangement. Electron spectrometry spans many orders of magnitude in energy, from less than 1 eV to ~100 keV. The range above 10 keV is largely the domain of magnetic spectrometers; the range below 1 keV is the domain of electro-static spectrometers; and the range in between is covered equally well by magnetic and electrostatic spectrometers, although magnetic instruments were employed in the past and electrostatic instruments might currently be given preference. Electron spectrometry encompasses many areas of research, both physical and chemical in nature, both fundamental and applied in scope. The field has been subdivided into several categories : photoelectron, Auger-electron, and electron energy-loss spectrometry, to name the most important. A number of books and reviews on the various aspects have appeared, specifically: Burhop (1952), Burhop and Asaad (1972), and Parilis (1969) on the Auger effect; Siegbahn et al. (1967, 1969) and Hagström and Fadley (1974) on electron spectrometry with emphasis on chemical applications; Palmberg (1972) on the application of the Auger effect in technology; Sevier (1972) on electron spectrometry in general; Ogurtsov (1972), Rudd and Macek (1972), and Garcia et al. (1973) on electron spectra in ion- 36 M. 0. Krause atom collisions; Bambynek et al. (1972) on fluorescence yields and transi-tion rates; and Massey (1971) on electronic phenomena. In this chapter, the potential and capability of electron spectrometry to delineate atomic excitation and deexcitation processes are emphasized and discussed with the aid of representative examples.
eBook - PDF- J.M. Walls, Robin Smith(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
Because of these chemical effects, the Swedish group coined the term electron spectroscopy for chemical analysis for their technique and the now famous acronym of ESCA. Although the term ESCA is still widely used today and indeed forms part of the model designation of several commercial spectrometers, it has largely been super-seded by the more spectroscopically exact term of XPS. Although in practice the terms are interchangeable, ESCA encompasses both X-ray induced photoemission (XPS) and Auger emission (X-AES). In 1966 Siegbahn published results that indicated the surface specificity of XPS 8 ; although not explicit in terms of analysis depth, they seemed to indicate the sampling depth of the XPS experiment to be less than 10 nm. Thus the scene was set for the development of a technique which not only provided identification of the chemical state of a material but also gave a surface specific rather than a bulk analysis. Much of the pion-eering work of Siegbahn's group on the XPS analysis of solid organic and inorganic materials was published in a single volume in 1967 1 . A companion volume on ESCA applied to free mol-ecules was produced a year or so later 9 . Today X-ray Photoelectron Spectroscopy (XPS) is applied to a wide range of materials, in order to obtain a variety of analytical information. Although XPS can be applied to gases, liquids and solids, in this article we shall only be concerned with its relevance to solid surfaces and the contribution that the data produced can make in the fields of chemistry, physics and materials science. XPS is widely used in both surface science, that is the gleaning of knowledge concerning the composition and crystallography of simple (often single crystal) surfaces and the modification of said surfaces in a well controlled manner, and surface analysis ; the acquisition of analytical information concerning the outer sur-face layers of a, material.
eBook - PDFHandbook of Surface and Interface Analysis
Methods for Problem-Solving, Second Edition
- John C. Riviere, Sverre Myhra(Authors)
- 2009(Publication Date)
- CRC Press(Publisher)
61 3.13 ISS ......................................................................................................................................... 61 3.13.1 Dedicated Instruments ............................................................................................. 61 References ........................................................................................................................................ 62 3.1 INTRODUCTION The most commonly used surface spectroscopic techniques are x-ray Photoelectron Spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), Auger electron spectroscopy (AES), and ion scattering spectroscopy (ISS). XPS and UPS are similar techniques and can be grouped under pho-toemission spectroscopy (PES). All four techniques are used widely for the study of solid surfaces both in fundamental scientific studies and in applied studies of polymers, ceramics, heterogeneous catalysts, metals and alloys, semiconductors, nanoparticles, biomaterials, etc. They can provide information about composition, chemical state, electronic structure, and geometrical structure. Detailed reviews have been presented previously [1,2]. 3.2 PHYSICAL PROCESSES In this section, the underlying physical processes are outlined and the nature of the information provided by the resultant spectra is discussed. 3.2.1 P HOTOEMISSION In PES, either x-rays (XPS) or UV photons (UPS) strike the surface of a sample in an ultrahigh vacuum (UHV) environment. Electrons are emitted as shown schematically in Figure 3.1A. The emission process has been described by Berglund and Spicer [3] in a three-step model, in which the first step involves absorption of an x-ray or UV photon and promotion of an electron from its ground state to the final state above the fermi level. The final state lies within the potential field of the solid FIGURE 3.1 Schematic diagrams of the (A) photoemission, (B) Auger, and (C) fluorescence processes.
eBook - PDF- Hatsuo Ishida(Author)
- 2013(Publication Date)
- Newnes(Publisher)
XPS/ESCA Energy analyzer Computer Sample Detector Figure 1.1 X-ray photoelectron spectrometer the applications of the technique in the investigation of the surface chemistry of polymer-based composites and composite components. These two names, XPS and ESCA, are commonly used to indicate this surface spectroscopic technique. The designation XPS arises from early studies of the photoelectric effect, where photons with energies in the X-ray region were used to promote ionization. 2 ' 3 The term ESCA was introduced by Siegbahn and coworkers 2 to emphasize the potential use of electron spectroscopy for elemental and chemical analysis. 1.2 Fundamental Principles: XPS and ESCA The fundamental event associated with XPS/ESCA measurements is photoioniza-tion. 2 -4 In the measurements, a sample maintained in a high vacuum is irradiated with soft X rays (usually Mg Ka, 1253.6 eV, or Al Ka, 1486.6 eV), photoionization occurs, the kinetic energy of the ejected photoelectrons is measured, and output data and information related to the number of electrons detected as a function of energy are produced. Interaction of the soft X-ray photon with composite material results in ionization from core and valence electron energy levels of the elements in the surface of the composite. Determination of the kinetic energy of the photoe-jected electrons permits identification of the elemental composition of the com-posite surface. To accomplish the determination, the essential components for an XPS/ESCA spectrometer include an X-ray source, a sample holder, an electron energy analyzer, an electron detector, and associated data acquisition and output devices (a computer). Figure 1.1 shows these components for a spectrometer that employs electrostatic energy analysis. Contemporary XPS/ESCA spectrometers may include the capability of producing monochromatic X rays.
eBook - PDF- W Delgass(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
If that sample is a single crystal, however, the probability of electron escape from the solid has an added angular dependence due to diffraction and channelling in specific crystal directions [115]. The peaks in the intensity versus Θ plot caused by the diffraction/channelling phenomena attest to the order in the surface layer and can be used for struc-tural analysis [109, 115]. The final type of angular dependence originates in the directionality of bonding at well defined surfaces. The effect has been most widely used, as mentioned earlier, in conjunction with uv and synchrotron radiation which show the greatest sensitivity to the bonding electrons. It has also been used in XPS to probe the symmetry of the 3d orbitals in Cu single crystals [116]. 8.4 Experimental Equipment and Procedures In XPS experiments we generally seek to establish line positions to within 0.1 eV. In order to maximize resolution and therefore differentiation of chemical states, we want linewidths to be as narrow as possible. High sensi-tivity, needed to examine submonolayer quantities of surface species and to minimize data collection time, requires high count rates at high signal to noise ratio. Achievement of these experimental capabilities demands sophis-ticated equipment. Several excellent spectrometers are available commercially at a cost of ~ 80-150 thousand dollars, fully outfitted. Additional special features have been included on some custom-built machines. In the sections that follow we describe the general characteristics of equipment design, data collection and processing, and experimental procedures and strategy. A. Equipment The schematic diagram in Fig. 8-1 shows four main components of an x-ray photoelectron spectrometer, the x-ray source, the sample, the ß-spectrometer or electron energy analyzer, and the detector. To complete the list we need only add a control and data collection device. The simplest x-ray source is a conventional x-ray tube.
eBook - PDFCharacterization of Metal and Polymer Surfaces V1
Metal Surfaces
- Lieng-Huang Lee(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
The insert is an emission hemisphere which shows (shading) the angular properties of the hydrogen-induced structure. (From Anderson and Lapeyre, Ref. 63). periodicity is not altered, chemisorbed atoms might modify the photoemission current through their influence on the scattering amplitudes in the Umklapp processes. 63 In terms of special selected examples, a general overview of the present status of ultraviolet photoemission spectroscopy (UPS) as a tool for the study of chemisorptive bonding on metals is presented. UPS-spectroscopy is a simple and versatile experimental probe with minimal perturbation on the nature and configuration of the solid surface. The UV-photon beam is nondestructive to surface chemical complexes. Reasonably useful energy resolution (0.1-0.3 eV) is attainable using relatively simple energy analyzers. Primary applications to chemisorption include energy level measure-ments and chemical fingerprinting of the bonding orbitals in chemisorbed species. Photoemission is manifestly a multielectron process and phenomena such as final-state relaxation and electron shake-up complicate theoretical descriptions of absolute binding energies. Further uncertainties due to molecular distortion can arise in systems showing strong chemisorption. Despite these constraints UPS provides one of the best single techniques for the study of bonding at surfaces. When coupled with other complimen-tary surface-sensitive techniques, it is a promising approach to the probing of the detailed microscopic nature of heterogeneous SUMMARY AND FUTURE PROSPECTS 4 6 2 Thor Rhodin and Charles Brucker surface systems. There are some recent rather exciting applications which utilize the dependency of orbital ionization features on the angle of detection as well as on the energy and polarization of the in-cident radiation.
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