Electron ejected

XPS X-ray photoelectron spectroscopy [131-137] Monoenergetic x-rays eject electrons from various atomic levels the electron energy spectrum is measured Surface composition, oxidation state... 

Coincidence experiments explicitly require knowledge of the time correlation between two events. Consider the example of electron impact ionization of an atom, figure Bl.10.7. A single incident electron strikes a target atom or molecule and ejects an electron from it. The incident electron is deflected by the collision and is identified as the scattered electron. Since the scattered and ejected electrons arise from the same event, there is a time correlation... 

Figure Bl.10.7. Electron impact ionization coincidence experiment. The experiment consists of a source of incident electrons, a target gas sample and two electron detectors, one for the scattered electron, the other for the ejected electron. The detectors are coimected tlirough preamplifiers to the inputs (start and stop) of a time-to-amplitiide converter (TAC). The output of the TAC goes to a pulse-height-analyser (PHA) and then to a nuiltichaimel analyser (MCA) or computer.

S NH neutral plus an ejected electron in which the NH is in its v = 0 vibrational level (no higher level... 

Because NH has an electron affinity of 0.4 eV, the total energies of the above two states can be equal only if the kinetic energy KE carried away by the ejected electron obeys... 

Ions are directed onto the first plate (dynode) of an electron multiplier. The ejected electrons are accelerated through an electric potential so they strike a second dynode. Suppose each ion collision causes ten electrons to be ejected, and, at the second dynode each of these electrons causes ten more to be ejected toward a third dynode. In such a situation, arrival of just one ion causes 10 X 10 = 10 = 100 electrons to be ejected from the second dynode, an amplification of 100. Commercial electron multipliers routinely have 10, 11, or 12 dynodes, so amplifications of 10 are... 

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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When M is an atom the total change in angular momentum for the process M + /zv M+ + e must obey the electric dipole selection mle Af = 1 (see Equation 7.21), but the photoelectron can take away any amount of momentum. If, for example, the electron removed is from a d orbital ( = 2) of M it carries away one or three quanta of angular momentum depending on whether Af = — 1 or +1, respectively. The wave function of a free electron can be described, in general, as a mixture of x, p, d,f,... wave functions but, in this case, the ejected electron has just p and/ character. 

In molecules, also, there is no restriction on the removal of an electron. The main difference from atoms is fhaf, since fhe symmefry is lower, fhe MOs fhemselves are mixfures of s,p, d,f,... AOs and fhe ejected electron is described by a more complex mixture of s,p, d,f,... character. 

An example of a hole color center is smoky quart2 [14808-60-7]. Here itradiation (either produced by nature or in the laboratory) of Si02 containing a trace of A1 ejects an electron from an oxygen adjacent to the A1 or, in customary nomenclature [AlO ] — [AlO ]" + e the ejected electrons are... 

Fig. 17. The analysis depth in XPS varies as a function of the take-off angle or the angle between the sample surface and the direction in which the ejected electrons are propagating.

EXAFS is observed as a modulating change in the absorption coefficient caused by the ejected electron wave back-scattering from the surrounding atoms, resulting in interference between ejected and back-scattered waves. It is defined as ... 

The kinetic energy of the ejected electrons increases linearly with the frequency... 

What does tins equation tell us Because the kinetic energy of the ejected electrons varies linearly with frequency, a plot of the kinetic energy against the frequency of the radiation should look like the graph in Fig. 1.17, a straight line of slope h, the same for all metals, and have an extrapolated intercept with the vertical axis at — horizontal axis (corresponding to zero kinetic energy of the ejected electron) is at fI>/6 in each case. 

FIGURE 1.17 When photons strike a metal, no electrons are ejected unless the incident radiation has a frequency above a value characteristic of the metal. The kinetic energy of the ejected electrons varies linearly with the frequency of the incident radiation. The inset shows the relation of the slope and the two intercepts to the parameters in Eq. 5. 

The velocity of an electron that is emitted from a metallic surface by a photon is 3.6 X 103 km-s. (a) What is the wavelength of the ejected electron (b) No electrons are emitted from the surface of the metal until the frequency of the radiation reaches 2.50 X i 016 Hz. How much energy is required to remove the electron from the metal surface ... 

The work function for chromium metal is 4.37 eV. What wavelength of radiation must be used to eject electrons with a velocity of 1.5 X 103 km-s ... 

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. 

We know v, the frequency of the radiation being used to bombard the molecules so, if we could measure the kinetic energy of the ejected electron, EK, we could solve this expression to find the orbital energy, orbjtai-... 

The ability of nuclear radiation to eject electrons from atoms and ions can be used to measure its intensity. Becquerel first gauged the intensity of radiation by determining the degree to which it blackened a photographic film. The blackening results from the same redox processes as those of ordinary photography, such as... 

The control of materials purity and of environmental conditions requires to implement physico-chemical analysis tools like ESC A, RBS, AUGER, SEM, XTM, SIMS or others. The principle of SIMS (Secondary Ion Mass Spectroscopy) is shown in Eig. 31 an ion gun projects common ions (like 0+, Ar+, Cs+, Ga+,. ..) onto the sample to analyze. In the same time a flood gun projects an electron beam on the sample to neutralize the clusters. The sample surface ejects electrons, which are detected with a scintillator, and secondary ions which are detected by mass spectrometry with a magnetic quadrupole. 

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