Photoelectron from photoelectric effect

X-ray photoelectron spectroscopy (XPS) is among the most frequently used surface chemical characterization teclmiques. Several excellent books on XPS are available [1, 2, 3, 4, 5, 6 and 7], XPS is based on the photoelectric effect an atom absorbs a photon of energy hv from an x-ray source next, a core or valence electron with bindmg energy is ejected with kinetic energy (figure Bl.25.1) ... 

Photoelectron spectroscopy involves the ejection of electrons from atoms or molecules following bombardment by monochromatic photons. The ejected electrons are called photoelectrons and were mentioned, in the context of the photoelectric effect, in Section 1.2. The effect was observed originally on surfaces of easily ionizable metals, such as the alkali metals. Bombardment of the surface with photons of tunable frequency does not produce any photoelectrons until the threshold frequency is reached (see Figure 1.2). At this frequency, v, the photon energy is just sufficient to overcome the work function

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Xps is based on the photoelectric effect when an incident x-ray causes ejection of an electron from a surface atom. Figure 7 shows a schematic of the process for a hypothetical surface atom. In this process, an incident x-ray photon of energy hv impinges on the surface atom causing ejection of an electron, usually from a core electron energy level. This primary photoelectron is detected in xps. 

One of the most direct methods is photoelectron spectroscopy (PES), an adaptation of the photoelectric effect (Section 1.2). A photoelectron spectrometer (see illustration below) contains a source of high-frequency, short-wavelength radiation. Ultraviolet radiation is used most often for molecules, but x-rays are used to explore orbitals buried deeply inside solids. Photons in both frequency ranges have so much energy that they can eject electrons from the molecular orbitals they occupy. 

Based on the photoelectric effect, electrons in evacuated tubes (photoelectrons) are released from a metal surface if it is irradiated with photons of sufficient quantum energy. These are simple photocells. Photomultipliers are more sophisticated and used in modem spectrophotometers where, via high voltage, the photoelectrons are accelerated to another electrode (dynode) where one electron releases several electrons more, and by repetition up to more than ten times a signal amplification on the order of 10 can be obtained. This means that one photon finally achieves the release of 10 electrons from the anode, which easily can be measured as an electric current. The sensitivity of such a photomultiplier resembles the sensitivity of the human eye adapted to darkness. The devices described are mainly used in laboratory-bound spectrophotometers. 

X-ray photoelectron spectroscopy (XPS) is based on the photoelectric effect. When a sample is irradiated with monochromatic X-rays, such as the K lines of Mg (1253.6eV) or Al (1486.6 eV), core-level electrons from the inner shells of atoms in the sample will be ejected from the sample to the surrounding vacuum. The kinetic energy, Er, of the emitted photoelectron is given by... 

When a photon of light hits the surface of a piece of metal, it may, if there is sufficient energy, eject an electron from the metal. Such an electron is called a photoelectron, and the mechanism is known as the photoelectric effect. The diagram at the right shows a setup for measuring the photoelectric effect. 

Figure 12.5 The photoelectric effect. Light with photons of energy hc/k approaches from the left, strikes the atoms in the metal, and ejects a photoelectron with a kinetic energy equal to the photon energy minus the work function of the metal. This demonstrates the particulate nature of light.

The potential curves of the adsorption of cesium on a CaF2 surface are given in Fig. 21, which shows that the curve for the ion represents an endothermic chemisorption. By the absorption of light of suitable wave length the system is transferred from minimum B to a point P of the upper curve and an electron is freed and may be drawn off as a photoelectron. The phenomenon of the selective photoelectric effect could be fully explained by this photoionization process (174). By thermal excitation the transfer can be effected at point electron emission of oxide cathodes. Point S is reached by taking up an amount of energy, which may be called the work function of the oxide cathode in this case but which is completely comparable with the energy of activation in chemisorption discussed in Sec. V,9 and subsequently. We shall not discuss these phenomena in this article but refer to a book of the author where these subjects are dealt with in detail (174) ... 

Photoelectron spectroscopy is based on Einstein s photoelectric effect. A photon can remove an electron from a molecule or material if the photon has an energy greater than... 

The photoelectric effect, first outlined by Einstein in the early 1900s, refers to the ejection of electrons from a surface due to photon impingement. However, it was not until the 1960s that this phenomenon was exploited for surface analysis - a technique referred to as X-ray photoelectron spectroscopy (XPS), or electron spectroscopy for chemical analysis (ESCA). This technique consists of the irradiation of a sample with monochromatic X-rays e.g., A1 Ka (1.487 keV), Mg Ka (1.254 keV), Ti Ka (2.040 keV)), which releases photoelectrons from the sample surface (Figure 131) Due to the short free mean path (EMP) of the photoelectrons in the solid, this technique provides compositional information from only the top 1-5 nm of a sample. 

The history of X-ray photoelectron spectroscopy or XPS is closely linked with the development of ideas on the structure of the atom. The discovery of the photoelectric effect by Hertz in 1887 showed that electrons could be separated from their constituent atoms under the effect of electromagnetic rays. 

FIGURE 4.15 Frequency and intensity dependence of the photoelectric effect. Only light above the threshold frequency can eject photoelectrons from the surface. Once the frequency threshold has been passed, the total current of photoelectrons emitted depends on the intensity of the light, not on its frequency. 

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