Photoelectricity

We use the X or gamma rays power penetrating to detect possible heterogeneities in inspected pieces. These rays are absorbed by the matter crossed, essentially by the photoelectrical effect, (fig. 02). 

Photoelectron spectroscopy provides a direct measure of the filled density of states of a solid. The kinetic energy distribution of the electrons that are emitted via the photoelectric effect when a sample is exposed to a monocluomatic ultraviolet (UV) or x-ray beam yields a photoelectron spectrum. Photoelectron spectroscopy not only provides the atomic composition, but also infonnation conceming the chemical enviromnent of the atoms in the near-surface region. Thus, it is probably the most popular and usefiil surface analysis teclmique. There are a number of fonus of photoelectron spectroscopy in conuuon use. 

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) ... 

The final technique addressed in this chapter is the measurement of the surface work function, the energy required to remove an electron from a solid. This is one of the oldest surface characterization methods, and certainly the oldest carried out in vacuo since it was first measured by Millikan using the photoelectric effect [4]. The observation of this effect led to the proposal of the Einstein equation ... 

Fowler R FI 1931 The analysis of photoelectric sensitivity curves for clean metals and various temperatures Phys. Rev. 38 45-56... 

Like sulphur, selenium has been used in the vulcanisation of rubber. It is also used in photoelectric cells. 

Because of it has great affinity for oxygen, the metal is used as a "getter" in electron tubes. It is also used in photoelectric cells, as well as a catalyst in the hydrogenation of certain organic compounds. 

The work function (p is the energy necessary to just remove an electron from the metal surface in thermoelectric or photoelectric emission. Values are dependent upon the experimental technique (vacua of 10 or torr, clean surfaces, and surface conditions including the crystal face identification). 

Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption). All methods of detection, whether the human eye or a modern photoelectric transducer, measure the transmittance of electromagnetic radiation. 

If photons of light of a suitable wavelength (usually ultraviolet or x-rays) impinge on a metal surface, electrons are emitted. This effect is photoelectric (or photovoltaic) and can be used to start a flow of electrons in a discharge tube. 

Irradiation of the cathode by the glow that starts (photoelectric release of electrons)... 

Another phenomenon that was inexplicable in classical terms was the photoelectric effect discovered by Hertz in f 887. When ultraviolet light falls on an alkali metal surface, electrons are ejected from the surface only when the frequency of the radiation reaches the threshold... 

The explanation of the hydrogen atom spectmm and the photoelectric effect, together with other anomalous observations such as the behaviour of the molar heat capacity Q of a solid at temperatures close to 0 K and the frequency distribution of black body radiation, originated with Planck. In 1900 he proposed that the microscopic oscillators, of which a black body is made up, have an oscillation frequency v related to the energy E of the emitted radiation by... 

Einstein, in 1906, applied this theory to the photoelectric effect and showed that... 

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

[Pg.289]

Phofoelectron spectroscopy is a simple extension of the photoelectric effect involving the use of higher-energy incident photons and applied to the study not only of solid surfaces but also of samples in the gas phase. Equations (8.1) and (8.2) still apply buf, for gas-phase measuremenfs in particular, fhe work function is usually replaced by fhe ionization energy l so fhaf Equation (8.2) becomes... 

Even though Einstein developed the theory of the photoelectric effect in 1906 photoelectron spectroscopy, as we now know it, was not developed until the early 1960s, particularly by Siegbahn, Turner and Price. 

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. 

Iodine cataly2es the conversion of amorphous selenium to the black, semiconducting metallic modification, and is used for this purpose in the manufacture of photoelectric cells and electric rectifiers (see Seleniumand selenium compounds). 

O ne. Air pollution (qv) levels are commonly estimated by determining ozone through its chemiluminescent reaction with ethylene. A relatively simple photoelectric device is used for rapid routine measurements. The device is caHbrated with ozone from an ozone generator, which in turn is caHbrated by the reaction of ozone with potassium iodide (308). Detection limits are 6—9 ppb with commercially available instmmentation (309). 

S. M. Ryvkin, Photoelectric Effects in Semiconductors, Consultants Bureau, New York, 1964. 

---