Chemistry
Ion and Atom Photoelectron Spectroscopy
Ion and atom photoelectron spectroscopy is a technique used to study the energy levels and electronic structure of atoms and ions. By measuring the kinetic energy of photoelectrons emitted from the sample, valuable information about the ionization energy and electronic configuration can be obtained. This method is widely used in chemistry to understand the behavior of atoms and ions in various chemical processes.
Written by Perlego with AI-assistance
Related key terms
1 of 5
10 Key excerpts on "Ion and Atom Photoelectron Spectroscopy"
- G. Svehla(Author)
- 2012(Publication Date)
- Elsevier Science(Publisher)
Inevitably, however, much excellent work is not even mentioned, and the illustrative applications reflect the bias of a physicist un-aware of many of the most typically chemical aspects of the subject: but an attempt has been made to develop the experimental aspects which seem to offer good prospects for future development. 2. Fundamental principles (A) PHOTOIONIZATION A N D PHOTOELECTRON SPECTROSCOPY When a beam of electromagnetic radiation or photons passes through a substance it will be attenuated by a number of processes. These processes may be characteristic of the individual atoms or molecules, or may also depend on the bulk properties of the sample if the molecules are associated with others as in a liquid or a solid. In this chapter, we are concerned only with the former, free atoms and molecules, which are ionized by the photon. The basic process is thus photoionization, which we represent by the equation. hv + Mi-* M+ +e(E) (1) References pp. 82—88 3 If the ionization energy or potential required to produce the final state, f, is e f — e i ? then, since the mass of the ion is large compared with that of the electron, conservation of momentum determines that the recoil energy of the ion is usually quite negligible for photons less than 100 eV. Thus the electron energy Ε is given by E = hv-(e f -e i ) (2) Recognising that the final state of the ion may have vibrational and rotational excitation of a particular electronic state, we can rewrite this as E = h p -I u -ΑΕ ν κ - AE rot (3) This is the basic equation used in photoelectron spectroscopy. The energy, E, probability of ejection (cross-section σ), angle of ejection of the photoelectron, and the excitation or species of the ion, depend on the molecule studied and the photon energy.
eBook - PDF- Harry G. Brittain(Author)
- 2006(Publication Date)
- CRC Press(Publisher)
3 X-Ray Photoelectron and X-Ray Fluorescence Spectroscopy Harry G. Brittain Center for Pharmaceutical Physics, Milford, New Jersey, U.S.A. X-RAY EMISSION SPECTROSCOPIES Up to this point, we have considered only the spectroscopy associated with tran-sitions from states characterized by core electron wave functions to other states that are equally well defined by different wave functions. This spectroscopy description is appropriate as long as the quantum numbers of the terminal state are relatively low in magnitude, but different phenomena become important when the quantum numbers become sufficiently large. At high quantum numbers, the energy spacings between higher excited states become successively smaller until the states effectively merge into a continuum. In fact, when an anharmonic potential energy term is used in the Hamiltonian operator, one finds that at sufficiently high quantum numbers dissociation of an excited electron may take place. This information is of great interest, because knowledge of the energy required to dissociate an electron would be directly related to the energy of the state from which it had been removed. The ionization potential of an electron is defined as the energy required for its removal from its orbital of occupancy. Originally, investigation of the magnitude of ion currents obtained as a function of the energy of an impacting species were used to deduce ionization potentials, but these studies gen-erally yielded information only on a few electrons in the valence shells. However, the ionization of electrons from atoms upon irradiation by X-rays is a phenomenon known as the photoelectric effect , the process being expressed as: A þ h n ! A þ þ e (1) 67 where h n is the energy of the impacting photon. One could study the characteristics of the ionized atoms (as is done in the practice of mass spectrometry), or one may study the characteristics of the ejected photoelectrons.
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- 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 - PDF- W Delgass(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Chapter 8 X-RAY PHOTOELECTRON SPECTROSCOPY 8.1 Introduction In 1905 Einstein explained that the energy of an electron ejected by the photoemission process could be calculated as the diiference between the energy of the photon hv and the binding energy of the electron in the target [1]. In the 1950s Siegbahn and co-workers began development of high reso-lution /^-spectrometers capable of measuring electron kinetic energies with a precision of 1 part in 10 4 [2]. With such devices and the Einstein equation, the Siegbahn group began to measure binding energies of core electrons by x-ray photoemission. The discovery that these energy levels shift as much as 6 eV with changes of chemical state of the atom being studied [3] precipitated rapid development of a technique known as Electron Spectroscopy for Chemical Analysis (ESCA) [4] or x-ray photoelectron spectroscopy (XPS). While early ESCA experiments dealt with core electrons from solids, Turner and coworkers carried out a parallel development of ultraviolet-induced photoemission (UPS) of valence electrons in gases [5]. X-ray photoelectron, ultraviolet-induced photoemission, and Auger elec-tron spectroscopy (AES) comprise a family of induced electron emission techniques which is a rich source of new information on surfaces. In this 267 268 8. X-Ray Photoelectron Spectroscopy chapter we focus on XPS of core electrons, although some of the character-istics of UPS and AES will be described for comparison. Since the entire field of electron spectroscopy of surfaces is still developing very rapidly, the power and limits of XPS are not yet fully established. In the discussion of theory, experiment, and application to follow, we will try to diiTerentiate areas which are well established from those which are still the subject of active research. Before embarking on a detailed description, however, we consider the general characteristics of x-ray photoelectron spectroscopy and its attributes as a tool for research on surfaces.
- 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- D. Phil Woodruff(Author)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
5 Methods of Determining Surface Electronic Structure 5.1 Ultraviolet Photoelectron Spectroscopy In Chapter 2 the technique of X-ray photoelectron spectroscopy was introduced as a means of accessing atomic core states in a solid surface in order to determine surface composition. In simple terms, when a photon of energy hν impinges on the surface, photoelectrons emerge from the surface with an energy spectrum contain- ing peaks corresponding to all the occupied states with binding energy less than hν (see Fig. 2.1). The highest kinetic energy electrons arise from emission from the valence states, so this part of the X-ray photoelectron spectrum may be expected to have a peak shape that reflects the DOS of the occupied valence electron states, albeit modified by different cross-sections for states of different orbital character; this is discussed further in section 5.5.1. Of course, if one is interested only in these shallowly bound states, it is more appropriate to use significantly lower photon energies than those (~1200–1500 eV) typically used in XPS; lower photon energies lead to higher photoemission cross-sections and make it easier to achieve higher energy resolution. This approach leads to the traditional laboratory-based technique of ultraviolet photoelectron spectroscopy (UPS). This is typically performed using a He gas discharge line source which can be operated to maximise the output of either He I (21.2 eV) or He II (40.8 eV) radiation, although other inert gas species (providing somewhat lower energies) are also sometimes used. However, as discussed below, there are important advantages that arise if the photon energy is more widely and continuously variable, so all the most incisive applications of this basic technique now rely on the use of synchro- tron radiation. Using these lower photon energies one can access the occupied band states of a clean solid surface as well as the bonding orbital states of adsorbed molecules.
eBook - PDFMass Spectrometry
Volume 5
- R A W Johnstone(Author)
- 2007(Publication Date)
- Royal Society of Chemistry(Publisher)
A. Miller, J. Chem. Phys., 1977,67, 1790. 24 Mass Spectrometry The kinetic energy released into the ionic fragments, and by conservation of momentum laws into the neutral fragments, may be measured by ion-beam deflection, electric sector analysis, and retarding potentials which measure ion velocities directly or indirectly by peak shape (of both ‘normal’ and metastable ions) analysis. Work with metastable ions has the disadvantage that the actual amount of excess of internal energy is imprecisely defined although each metast- able ion lifetime ‘window’ defines a small spread in excess of energy which, in suitably conducted experiments will lie near the thermochemical threshold for reaction. Ions with precisely defined excess of internal energy are formed by ionization with monoenergetic photons. An ion of ionization energy, I, will contain an excess of energy, E* = hv -I - KE(e-), hv is the energy of the incident photons and KE(e-) is the kinetic energy of the ejected electrons. The photoion-photo- electron coincidence method selects positive ions coincident with any ejected electron so that with KE(e-), hv, and I known, E* is obtained simply; by varying hv,E* can be varied. Peak shape analysis (distribution of ion flight-times is altered if ions fragment in transit, releasing kinetic energy) is used to obtain the kinetic energy release. The average kinetic energy release for the reaction, CH,? + CH3’+H’, has been measured and found to be very small at thresh01d.I~~ Above threshold, the ratio of kinetic energy released to total excess of energy in the ion was found to be a constant equal to 0.23 as against 0.12 predicted the~retically.’~~ The insignificant energy release at threshold is in keeping with a simple bond-breaking mechanism which is not expected to have a significant activation energy for the reverse reaction. The result confirms a slightly earlier result for this fragmentation.
eBook - PDFCharacterization of Metal and Polymer Surfaces V1
Metal Surfaces
- Lieng-Huang Lee(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
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. Improved angular-resolved photoemission spectro-meters are being developed to provide measurements that may allow determination of the geometrical orientation of adsorbed species and also to offer insight into the molecular orbital structure of the surface complex. Energy-dependent photoionization cross-section measurements of both adsorbed and gas phase molecules, exploiting synchrotron radiation, puts identification of surface energy levels on a firmer basis. The use of polarized radiation to enhance sur-face features and possibly to give indications of molecular orbital structure and orientation also promises to be very useful. At this time it is premature to claim an understanding of the microscopic electronic structure of solid surfaces with this type of technique. Close interplay, however, with theoretical methods can realisti-cally provide a more accurate and improved understanding of elec-tronic and geometric properties governing chemical reactions on surfaces. Major questions have also been raised as to the connection between the interpretation of simple chemical reactions on well-defined single-crystal metals and the comparative usefulness of different approaches to the electron spectroscopy of practical catalytic materials. It has been pointed out 68 that the objective of determining electronic and atomic structure can be useful but difficult when a catalytic reaction takes place because the molecules are both larger and more complicated, co-adsorption or sequential chemisorption can occur, and order-disorder phenomena are often involved. The usefulness of photoemission studies to evaluate the role of chemical bonding and molecular properties in these systems has, as yet, not been explored.
eBook - PDFRadiation Research
Biomedical, Chemical, and Physical Perspectives
- Oddvar F. Nygaard, Howard I. Adler, Warren K. Sinclair, Oddvar F. Nygaard, Howard I. Adler, Warren K. Sinclair(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
For instance, for ionization one has: Ai = ft? *?i(E) (R/E)(df/dE)dE, (R = Rydberg energy) (4) where 771 (E) is the fraction of the continuous optical oscillator strength df/dE that leads to ionization and the integral extends over the entire photo-absorption continuum from the IP up. From eqs. (2) and (3) it is immediately apparent that a simple plot of σΕ 0 vs. In E 0 (Bethe plot) tells us the nature of the process under study (see Fig. 3a). What can one learn about superexcited states from high energy cross sections? Firstly, suppose the spectrum of df/dE of a molecule is known, but not the ionization efficiency. Then Ai, as obtained in an electron impact experiment (eq. (2)), can be compared with (e.g. (75)) J7 p (R/E)(df/dE)dE The ratio between the two quantities, sometimes written as 17, differs from unity due to neutral excitations, that are excitations to superexcited states which do not auto ionize but decay into neutral fragments (see Table 1 ). In other words, TABLE 1 Ai f lp (R/E)(df/dE)dE Molecule electr. impact (16) photoabsorption (75) co 2 5.75 ± 0.07 5.97 H 2 o 3.24 ±0.15 3.21 NH 3 3.58 ± 0.35 3.98 C H 4 4.23 ±0.13 4.55 C 2 H 6 6.80 ± 0.36 7.38 C H 3 O H 6.22 ±0.18 7.56 C 2 H 5 O H 9.94 ± 0.27 11.8 2 Or the appropriate relativistic formulae (5) in case Eo is above roughly 10 keV. 208 RADIATION RESEARCH one obtains a spectral average of the contribution of non-ionizing superexcited states (averaged in a way that is relevant for electron impact); or rather, a lower limit to that contribution because of the competition between autoionization and neutral dissociation. Secondly, it is evident that of these two alternate decay routes mentioned, the second (dissociation) will be affected much more strongly than the first by changes in mass of the fragment. Indeed, strong isotope effects have been observed in ionization and dissociation cross sections (16—18).
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
