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- X-Ray Photoelectron Spectroscopy: An Introduction to Principles and Practices
- X-ray photoelectron spectroscopy : an introduction to principles and practices
- Modern ESCA : the principles and practice of X-Ray photoelectron spectroscopy
X-ray photoelectron spectroscopy XPS is a surface-sensitive quantitative spectroscopic technique based on the photoelectric effect that can identify the elements that exist within a material elemental composition or are covering its surface, as well as their chemical state , and the overall electronic structure and density of the electronic states in the material. XPS is a powerful measurement technique because it not only shows what elements are present, but also what other elements they are bonded to.
X-Ray Photoelectron Spectroscopy: An Introduction to Principles and Practices
The amount of electrons having escaped from the sample without energy loss is typically measured in the range of 20 to eV. The data is represented as a graph of intensity versus electron energy.
Due to the impact of the primary beam, the atoms in the sample are ionised, and electrons are liberated from the surface, either as a result of the photoemission process XPS , or of the radiationless de-excitation in the Auger electron emission process AES. As a consequence of that, one of the main differences is the lateral resolution of the two methods.
Since there is a continuous evolution in the design of the equipment and the performance of the techniques, it is difficult to express an absolute value for it, but for AES the lateral resolution typically is situated in the 10 to nm range, while by means of XPS only a lateral resolution of a few to m can be reached.
In both methods low energy electrons are measured, giving rise to comparable depth and sensitivity values, which are respectively in the order of nanometers see Fig. This type of measurements is necessarily performed under high vacuum conditions, and only samples restricted in size can be analysed.
From this point of view, XPS and AES cannot be considered as non-destructive techniques, although the analyses themselves are not destructive in nature. On the other hand, thanks to their spatial resolution, a small amount of material suffices for the analysis.
Moreover, samples can be analysed with XPS and AES in the as-received condition, and can in many cases be put back in their original position afterwards. In this chapter, we try to give practical information on XPS and AES, so that the reader can understand the basic principles, the characteristics and the potential of the methods and their instrumentation. More detailed information may be found in dedicated books. Strongly recommended are e.
Briggs and M. Seah , T. Carlson , J. Watts , B. Agius et al. Thompson et al. Nefedov . More practical information, e. At the end of this chapter, a paragraph is dedicated to a literature overview of the use of XPS and AES for the analysis of cultural heritage materials. An electron is ejected from an atomic energy level by an X-ray photon, mostly from an Al-K or Mg-K primary source, and its energy is analysed by the spectrometer.
The XPS process is schematically represented in Fig. The characteristic parameter for the electron is its binding energy. The relation between these parameters is given by Eq. From Eq. Each element has a characteristic electronic structure and thus a characteristic XPS spectrum.
In the spectrum, a number of peaks appears on a background. The observed peaks can be grouped into three types : peaks originating from photoemission i from core levels, ii from valence levels at low binding energies 0 to 20 eV and iii from X-ray excited Auger emission between and eV.
Although valence level spectra have their analytical value e. The main information comes from the core level peaks and the Auger peaks. As can be seen in Fig. The nomenclature employed to describe XPS and AES features is based on the momenta associated with the orbiting paths of electrons around atomic nuclei, indicated by the quantum numbers n, l, j.
And yet the translation into the notation is different for both techniques. Both notations are listed in Table 2. In the case of Ag, the Al-K radiation is only energetic enough to probe the core levels up to the 3s shell. The non s-level peaks are doublets. More details on the difference in energy between the two states and on their relative intensities can be found in .
It is also noted that the core level peaks have different intensities and widths. The relative intensities are governed by the ionization efficiencies of the different core shells, designated by ionization cross section. Table 2. The Auger peaks in the spectrum, MNN peaks in the case of Ag, originate from the decay of the ionized atoms to their ground state.
The principle of the Auger emission is discussed in the next paragraph. The analyzed electrons are not the emitted core electrons, but the Auger electrons that are ejected as a consequence of the return of the ionized atom to its ground state. For the example shown here, a hole is created on the K level in the initial ionization step. This requires a primary energy greater than the binding energy of the electron in that shell.
For the ionization to be efficient, a primary energy of about 5 times the binding energy is taken. In practice, typical primary energies are 5 and 10 keV.
The hole can be produced by either the primary beam, or the backscattered secondary electrons The atom relaxes by filling the hole with an electron coming from an outer level, in the example shown as L1. The emission of an X-ray at that energy may occur or the energy may be given to another electron, either in the same level or in a more shallow one, as is the case in the example, to be ejected. The first of the two competing processes is X-ray fluorescence, the second Auger emission.
Fortunately, the probability for Auger emission is much higher for core levels with binding energies below about 2 keV, as illustrated in Fig. An AES transition is written as ABC, where A indicates the level of ionization, B the level where the second electron involved in the transition comes from, and C the level from which the Auger electron is emitted.
The Auger transition represented in Fig. Electrons originating from the valence band, are often denoted by V. More details on the notations and the corresponding electronic configurations can be found in .
In good approximation Eq. For this reason, AES spectra are always plotted on a kinetic energy scale. The AES peaks are superimposed on an important background of different types of secondary electrons. This is the reason why, in many cases, AES spectra are represented in the differentiated form. This is because both techniques do not use the same energy scale : kinetic energies for AES spectra against binding energies for XPS spectra.
The energy positions of X-ray and electron induced Auger transitions differ, as shown by Eq. This allows the two types of peaks to be easily distinguished.
As is the case for XPS peaks, the relative intensities of AES transitions are governed by their respective core shell ionization efficiencies due to the primary electron beam. Yet, for AES the situation is more complex, since there is additional ionization due to back-scattered electrons see Fig.
Ip is peak intensity due to primary ionization, Ib to back-scattered induced ionization. The back-scattering factors depend both on the energy and the angle of incidence of the primary beam, and they influence the intensities as well as the spatial distribution of the detected Auger electrons as illustrated in Fig. The sample consists of a 40 nm thick Al layer on Au.
The Al KLL peak is shown. This is related to the complex multiplet splitting due to the number of final states after the transition that are permitted see Table 2. The KLL series, for example, consists of five components, i. Two elements contribute to the peak broadening in solid species : peak overlap in the multiplet structure, and solid state peak broadening.
As for most techniques, the system is operated and controlled by a computer, usually provided with software allowing mathematical treatment of the An example of a full set-up is given in Fig.
In Fig. By the introduction of field Emission AES, the level of 10 nm can now be reached. The lateral resolution of XPS currently reaches the level of a few micrometers, and XPS mapping facilities are becoming more frequently available than in the past. In the following paragraphs, the most essential parts of the instrumentation will be discussed in more detail.
The reason for this is twofold. The low energy electrons are elastically and non elastically scattered by residual gas molecules leading to a loss of intensity and of energy so that not only the intensity of the peaks is affected but also the noise in the spectrum increases.
The second reason is that lowering the vacuum level to e. A vacuum of torr allows measurements to be carried out for about an hour before a monofilm is formed. Even in the case of a vacuum of torr, typically carbon peaks are found as a result of surface contamination. Therefore, often the sample is cleaned by a slight sputtering, prior to the analysis. The sputtering technique will be discussed in paragraph 2. The high vacuum level imposes high requirements on the used materials.
Stainless steel is often selected for the fabrication of the analysis chamber and the joints are usually made of Cu rings.
X-ray photoelectron spectroscopy : an introduction to principles and practices
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Modern ESCA : the principles and practice of X-Ray photoelectron spectroscopy
The book first introduces the reader to the language and concepts used in this field and then demonstrates how these concepts are applied. Including how the spectra are produced, factors that can influence the spectra all initial and final state effects are discussed , how to derive speciation, volume analysed and how one controls this includes depth profiling , and quantification along with background substraction and curve fitting methodologies. Henry Cloud book. The nature of the question says that you might not be cut out for software engineering. She is just blond white trash and the white people who are as blasphemous and evil as she is certainly love the chaos and destructive behavior exhibited by her.
This book emphasizes the use of core level and valence band binding energies, their shifts, and line widths. Read more Please choose whether or not you want other users to be able to see on your profile that this library is a favorite of yours.
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