Electron photoelectric emission

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). 

A Degarive discharge electrode attracts positive ions and forces them to impact on its surface. These impacts provide an addirional source of electrons which contribute to the process. Ultraviolet light generated by the cormu glow causes photoelectric emission of electrons from the electrode surfaces, which further enhances the formation of free electrons. 

The electronic interaction is small if the metal used as an adsorbent has a work function low in relation to the polarizability of the adsorbed atoms. On adsorption of sodium atoms on an aluminum surface of = 4.08 volts, for instance, Brady and Jacobsmeyer (56) obtained a noticeable increase of the photoelectric emission only after five atom layers of sodium had been condensed. In this case the alkali layer itself and not the metal of the sublayer emitted the electrons. 

Fig. 18. Influence of molecular (B) and atomic (C) oxygen on a Pt surface covered with H atoms, investigated by the change of photoelectric emission. Ordinate photoelectric yield I in electrons per light quantum. X = 302.2 rn/i T = 293°K. (a) O2 admitted, po2 = 0.34 mm. Hg (6) O2 pumped off (c) O2 admitted, poz = 10 mm. Hg, Pt spiral kept at yellow heat for 2 min. (d) O2 pumped off [according to (58)).

When such a hydrogen-covered surface is bombarded with electrons of low energy (10 amp., 20 to 300 volts) (2a), both types of crystallites will be struck, The work function of crystallites II islowered, because their H2 molecules decompose into atoms under bombardment that of crystallites I is increased, because their H atoms are shot off. If the first effect predominates, the photoelectric emission of the platinum surface increases if the second effect prevails it decreases. The combination of both effects is seen in the results shown in Fig. 21. By adsorption of hydrogen, the sensitivity is increased D E). The electron bombardment causes at the beginning a further increase (F — F), because the H2 molecules decompose into atoms the later bombardment causes the photoemission to decrease again (F — G), because H atoms are shot off. The work functions, in volts, in the different states are... 

The lone electrons of the 0 atom in the H2O molecule can also become part of the electron gas in the metal surface and reduce its work function. So Schaaff (75) observed an increase of the photoelectric emission of platinum in the presence of water vapor. On the other hand an adsorbed layer of H2O molecules on the surface of a thin nickel film decreases the electric resistance of the film (18). 

More easily to be understood are the effects observed when ir electrons are present in the adsorbed molecule. Figure 28 shows the change of the photoelectric emission of a platinum surface covered with benzene (76). The benzene was contained in a capsule, which could be smashed magnetically (see F in Fig. 2). The tube, G in Fig. 2, was cooled by liquid air. At the points of the curve marked with arrows, the cooling of G was interrupted for 1 or 2 min., so that a small quantity of benzene molecules might be adsorbed at the platinum surface. The sensitivity increased (Fig. 28) at first and then decreased after passing a maximum, which was reached in the vicinity of the monomolecular covering (B, C in Fig. 28). 

The absorbed photons transmit their energy hv to electrons in the lattice of the solid near the surface. If the photon energy is sufficiently high, photoelectric emission is observed. This method of direct observation of the excited electrons enables information to be gained about the nature of the excited electrons and the initial states of the electron transitions. Thus, a complete analysis of the energy states of a semiconductor surface becomes possible. 

By measuring a physical process related to the electron distribution in the surface, it may be possible to link the photosorption process at the surface with a certain electron transition in the surface layer of the catalyst lattice. Photoelectric emission, thermal afterglow, photoconductivity and Hall effect, changes in surface potential or electron spin resonance are such processes. 

Like chemical actinometers, photocells have to be calibrated against a thermopile-galvanometer system this has to be done frequently as there tends to be some variation with time. Under these conditions, they can be used to measure the absolute intensity of monochromatic light. The cell best suited for photochemical studies is the photoemissive type, which operates via the photoelectric emission of electrons from an irradiated surface. The metallic cathode is mounted either in a vacuum or in a small pressure of one of the inert gases. The cell may involve a single phototube or a multielement photomultiplier. An amplification of about 10 is achieved with the latter. 

Electronic conduction Thermionic emission Photoelectric emission Photoconduction. Q QI2+X Q+X Q R RI2+X R+X R... 

He was awarded the Nobel Prize in physics in 1921A.D. for applying Planck s Quantum Theory to the explanation of the photoelectric emission of electrons. 

Practically inexhaustible possibilities are latent in the methods based on the external and internal photoelectric effects excited by x rays and light of various wavelengths. It is worth mentioning specially the methods of electron (p) spectroscopy, cold emission, photoelectric emission, and photoelectron spectroscopy, the last being used widely for the purpose of chemical analysis. 

Bandis C, Pate BB. Photoelectric emission from negative-electron-affinity dia-mond(lll) surfaces exciton breakup versus conduction-band emission. Phys Rev B 1995 52 12056-12071. 

The one-step photoelectric emission can produce the molecular ion in a wealth of electronic, vibrational, rotational, or translational excited states (Mj ", M2", ..., in addition to the ground state Mq. 

J. J. Thomson was able to show that the negative electric charge that leaves the zinc plate under the influence of ultraviolet light consists of electrons. The emission of electrons by action of ultraviolet light or x-rays is called the photoelectric effect. The electrons that are given off by the metal plate are called photoelectrons they are not different in character from other electrons. 

Since the electron transfer into the liquid is an adiabatic process, injection occurs into the delocalized state. In solid-state physics this energy pertains to the bottom of the conduction band. At temperatures T > 0 K, emission of electrons is already observed at light frequencies below Vq. It is a consequence of the thermal motion of the metal electrons. Electronic states above the Fermi level become occupied. The theory of photoelectric emission developed by Fowler (1931) takes this effect into account. Ihe normalized photoelectron current igi/Iph is written as... 

Electron sources include heated filaments, photoelectric emission, field emission, hollow cathodes and secondary electron emission. Ions can be extracted from a low pressure discharge plasma. After ions and electrons are generated, they are extracted through an... 

The X-ray spectrum observed in PIXE depends on the occurrence of several processes in the specimen. An ion is slowed by small inelastic scatterings with the electrons of the material, and it s energy is continuously reduced as a frmction of depth (see also the articles on RBS and ERS, where this part of the process is identical). The probability of ionizii an atomic shell of an element at a given depth of the material is proportional to the product of the cross section for subshell ionization by the ion at the reduced energy, the fluorescence yield, and the concentration of the element at the depth. The probability for X-ray emission from the ionized subshell is given by the fluorescence yield. The escape of X rays from the specimen and their detection by the spectrometer are controlled by the photoelectric absorption processes in the material and the energy-dependent efficiency of the spectrometer. 

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