Electron emission incident electrons

In a cascade process, one incident electron (e ) collides with a neutral atom ((S)) to produce a second electron and an ion ( ). Now there are two electrons and one ion. These two electrons collide with another neutral atom to produce four electrons and three ions. This process continues rapidly and — after about 20 successive sets of collisions — there are millions of electrons and ions. (The mean free path between collisions is very small at atmospheric pressures.) A typical atmospheric-pressure plasma will contain 10 each of electrons and ions per milliliter. Some ions and electrons are lost by recombination to reform neutral atoms, with emission of light. 

Edx is based on the emission of x-rays with energies characteristic of the atom from which they originate in Heu of secondary electron emission. Thus, this technique can be used to provide elemental information about the sample. In the sem, this process is stimulated by the incident primary beam of electrons. As will be discussed below, this process is also the basis of essentially the same technique but performed in an electron spectrometer. When carried out this way, the technique is known as electron microprobe analysis (ema). 

Figure 1 Schematic of an EDS system on an electron column. The incident electron interacts with the specimen with the emission of X rays. These X rays pass through the window protecting the Si (Li) and are absorbed by the detector crystal. The X-ray energy is transferred to the Si (Li) and processed into a dig-itai signal that is displayed as a histogram of number of photons versus energy.

The sensitivity of a photo-emissive cell (phototube) may be considerably increased by means of the so-called photomultiplier tube. The latter consists of an electrode covered with a photo-emissive material and a series of positively charged plates, each charged at a successively higher potential. The plates are covered with a material which emits several (2-5) electrons for each electron collected on its surface. When the electrons hit the first plate, secondary electrons are emitted in greater number than initially struck the plate, with the net result of a large amplification (up to 106) in the current output of the cell. The output of a photomultiplier tube is limited to several milliamperes, and for this reason only low incident radiant energy intensities can be employed. It can measure intensities about 200 times weaker than those measurable with an ordinary photoelectric cell and amplifier. 

The presence of adsorbed layers also affects the other parameters of the interaction between metastable atoms and a metal surface. Titley et al. [136] have shown that the presence of an adsorbed layer of oxygen on a W( 110) surface increases the reflection coefficient of helium metastable atoms. The reflection is of irregular nature and grows higher when the incidence angle of the initial beam increases. A series of publications [132, 136, 137] indicate that the presence of adsorbed layers causes an increase in the quantum yield of electron emission from a metal under the action of rare gas metastable atoms. 

Calculations using the CDW-EIS model [38] are shown to be in good accord with 40-keV protons incident on molecular hydrogen and helium, and at this energy both theory and experiment show no evidence of any saddle-point enhancement in the doubly differential cross sections. However, for collisions involving 100-keV protons incident on molecular hydrogen and helium the CDW-EIS calculations [39] predict the existence of the saddle-point mechanism, but this is not confirmed by experiment. Recent CDW-EIS calculations and measurement for 80-keV protons on Ne by McSherry et al. [41] find no evidence of the saddle-point electron emission for this collision. 

In Figs. 20 and 21 we compare the experimental and CDW-EIS results [38] for 40-keV H+ projectiles incident on H2 and He and for emission of electrons at 0° [doubly differential with respect to the electron polar angle of emission and the energy of the ejected electron d2a/dfldE (10-16 cm2 eV 1 sr 1)]. Both sets of results which did not require normalization are seen to be in very good accord, with the spectra being completely dominated by the ECC cusp. 

The unoccupied part of the density of states can also be measured, by a technique called Inverse UPS (Sometimes also referred to as BIS, Bremsstrahlung Isochromat Spectroscopie). Here a beam of low energy electrons falls on the surface, where they go into the unoccupied states and fall back to the Fermi level, under emission of a quantum hv. Measurement of this radiation as a function of the incident electron energy gives the density of unoccupied states. This technique falls outside the scope of this book. The reader is referred to the literature [5, 44],... 

Inherently connected to the interaction of ions with a solid is the emission of electrons from the sample. Typical yields are 0.1-0.2 electron per incident argon ion of 2 keV, and 0.2-0.5 at 5 keV [2], For the heavier krypton the values are about equal. Two factors contribute to electron emission ... 

The interaction of an electron with an atom gives rise to two types of X-rays characteristic emission lines and bremsstrahlung. The atom emits element-characteristic X-rays when the incident electron ejects a bound electron from an atomic orbital. The core-ionized atom is highly unstable and has two possibilities for decay X-ray fluorescence and Auger decay. The first is the basis for electron microprobe analysis, and the second is the basis of Auger electron spectroscopy, discussed in Chapter 3. 

Figure 3. ARUPS energy distribution curves taken with Hel radiation at normal incidence and an electron emission angle of 52" shown as a function of copper coverage. The intensity of the various curves has been normalized at the Fermi level Ef The individual curves are matched to their corresponding copper coverages in monolayers by the solid lines and the saturation behavior of the interface state at approximately —1.5 eV is identified by the dashed lines. (Data from ref. 8.) (Reprinted with permission from ref. 43. Copyright 1987 American Association for the Advancement of Science.)...

Some incident electrons will create inner shell vacancies as described above. The electrons ejected by the primary beam (photoelectrons) can be used analytically (in XPS) but are generally ignored in electron microscopy. The inner shell vacancy can de-excite via the Auger process (Auger electrons are also generally neglected in this application) or via the emission of characteristic X-rays, which are detected and which form the basis of the analytical operation of the electron microscope. 

Once electrons have been emitted by the photocathode, they are accelerated by an applied voltage induced between the photocathode and the first dynode (Uq in Figure 3.17). The dynodes are made of CsSb, which has a high coefficient for secondary electron emission. Thus, when an electron emitted by the photocathode reaches the first dynode, several electrons are emitted from it. The amplification factor is given by the coefficient of secondary emission, S. This coefficient is defined as the number of electrons emitted by the dynode per incident electron. Consequently, after passing the first dynode, the number of electrons is multiplied by a factor of 5 with respect to the number of electrons emitted by the photocathode. The electrons emitted by this first dynode are then accelerated to a second dynode, where a new multiplication process takes place, and so on. The gain of the photomultiplier, G, will depend on the number of dynodes, n, and on the secondary emission coefficient, 5, so that... 

Most of the energy associated with an incident x-ray or y-ray is absorbed by ejected electrons. These secondary electrons are ejected with sufficient energy to cause further ionizations or excitations. The consequences of excitations may not represent permanent change, as the molecule may just return to the ground state by emission or may dissipate the excess energy by radiationless decay. In the gas phase, excitations often lead to molecular dissociations. In condensed matter, new relaxation pathways combined with the cage effect greatly curtail permanent dissociation. Specifically in DNA, it is known that the quantum yields for fluorescence are very small and relaxation is very fast [6]. For these reasons, the present emphasis will be on the effects of ionizations. 

The photoelectric cross-section o is defined as the one-electron transition probability per unit-time, with a unit incident photon flux per area and time unit from the state to the state T en of Eq. (2). If the direction of electron emission relative to the direction of photon propagation and polarization are specified, then the differential cross-section do/dQ can be defined, given the emission probability within a solid angle element dQ into which the electron emission occurs. Emission is dependent on the angular properties of T in and Wfin therefore, in photoelectron spectrometers for which an experimental set-up exists by which the angular distribution of emission can be scanned (ARPES, see Fig. 2), important information may be collected on the angular properties of the two states. In this case, recorded emission spectra show intensities which are determined by the differential cross-section do/dQ. The total cross-section a (which is important when most of the emission in all direction is collected), is... 

As discussed in Section 2.2.2, an electron beam incident on a metal gives rise to the emission of characteristic X-rays from the metal. In electron microscopy, the elements present in the sample also emit characteristic X-rays. These are separated by a silicon-lithium detector, and each signal collected, amplified and corrected for... 

In this discussion, electrochemical reactions with semiconductors are referred to as thermal. The reason is that semiconductors are particularly sensitive to incident light, which stimulates electron emission and causes photocurrents to flow. They are... 

PHOTOEMISSION AND PHOTOMULTIPLIERS. Photoemission is the ejection of electrons from a substance as a result of radiation filling on it Photomultipliers make use of the phenomena of photoemission and secondary-electron emission in order to detect very low light levels The electrons released from the photocathode by incident light are accelerated and focused onto a secondary-emission surface (called a dynode). Several electrons are emitted from the dynode for each incident primary electron. These secondary electrons are then directed onto a second dynode where more electrons are released. The whole process is repealed a number of times depending upon the number of dynodes used, In this manner, it is possible to amplify the initial photocurrent by a factor of 10s or more in practical photomultipliers. Thus, the photomultiplier is a very sensitive detector of light. 

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