Ejection of photoelectrons

In X-ray photoelectron spectroscopy (XPS), a beam of soft X-rays with energy hv s. focused onto the surface of a solid that is held under an ultra-high vacuum, resulting in the ejection of photoelectrons from core levels of the atoms in the solid [20]. Fig. 15 shows an energy level diagram for an atom and illustrates the processes involved in X-ray-induced photoelectron emission from a solid. 

The singlet-triplet splitting of NH was determined experimentally by spectroscopy of neutral NH and by negative ion photoelectron spectroscopy (PES) of the NH anion. In the latter experiment, the anion NH is prepared in the gas phase and exposed to monochromatic ultraviolet (UV)-laser hght. This photolysis leads to ejection of photoelectrons whose kinetic energies ( k) are analyzed. As the energy... 

Photoelectrons are electrons that are ejected from a photosensitive surface by electromagnetic radiation. A photocurrent is the current in an external circuit that is limited by the rate of ejection of photoelectrons. 

The anomalous dispersion effect is associated with the ejection of photoelectrons from inner shell electrons in an atom. The normal scattering describes the interaction of all the electrons in the atom with the X-ray beam. The radial distribution of the electrons in an atom can be calculated using quantum mechanics, originally by Hartree s self-consistent field method (Hartree 1933). In figure 9.12 this distribution is given for rubidium, which has a K edge at 0.8155 A the mean radius for... 

Figure 1.6 is a schematic diagram of some of the important interactions between a solid specimen and an incident X-ray beam. These will result in diffracted X-rays, which have satisfied particular angular relations with lattice planes in the solid and will, therefore, contain crystallographic information. The interactions will also result in the ejection of photoelectrons, which will have energies equal to the difference between that of the incident photon and the binding energy of the electron in the state from which it is ejected. 

In X-Ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), mono-energetic soft X rays (usually A1 K alpha at 1489 eV or Mg K alpha at 1256 eV) bombard a solid sample in ultra high vacuum. One of the interaction processes occurring is the ejection of photoelectrons, according to the Einstein Photoelectric Law ... 

XPS X-ray photoelectron spectroscopy Absorption of a photon by an atom, followed by the ejection of a core or valence electron with a characteristic binding energy. Composition, oxidation state, dispersion... 

Using Equation (1.7) calculate the velocity of photoelectrons ejected from a sodium metal surface, with a work function of 2.46 eV, by ultraviolet light of wavelength 250 nm. 

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]

Figures 8.1(a) and 8.1(b) illustrate the processes involved in UPS and XPS. Both result in the ejection of a photoelectron following interaction of the atom or molecule M which is ionized to produce the singly charged M. ...

A common example of an Auger process involves the ejection of a photoelectron, as shown in Figure 8.23, from the K shell (i.e. a lx electron), with energy E, which is not considered further. Following the ejection of a if electron it is common for an electron from the L shell, specifically from fhe (or 2s) orbifal, fo fill fhe vacancy releasing an amounf of energy... 

X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (xps) and Auger electron spectroscopy (aes) are related techniques (19) that are initiated with the same fundamental event, the stimulated ejection of an electron from a surface. The fundamental aspects of these techniques will be discussed separately, but since the instmmental needs required to perform such methods are similar, xps and aes instmmentation will be discussed together. 

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. 

An XPS spectrum consists of a plot of N(E)/E, the number of photoelectrons in a fixed small interval of binding energies, versus E. Peaks appear in the spectra at the binding energies of photoelectrons that are ejected from atoms in the solid. Since each photoemission process has a different probability, the peaks characteristic of a particular element can have significantly different intensities. 

Three important detectors make use of the ionization, called here the initial ionization, that follows the absorption of x-rays by a gas and the ejection ol photoelectrons from the molecules involved. These photoelectrons subsequently ionize other molecules. The relatively large energy of the x-ray quantum thus leads to the production of a number of ion pairs, each consisting of an electron and a relatively immobile positive ion. if these ion pairs do not recombine, the extent of this initial ionization is determined by (and measures) the energy of the x-ray quantum. 

Absorption of X-ray radiation of energy well above the threshold for an X-ray transition will result in the ejection of a photoelectron since the initial unoccupied band stale to which the transition takes place will be above the vacuum level. The Kronig fine structure is due to oscillations induced in the absorption cross-section of the absorbing atom as a result of interference... 

Fig. 1 Excitation of a P atom leading to ejection of a photoelectron from the 2p orbital...

The fundamental process of the photoemission is the ejection of the photoelectrons by a material irradiated with photons of sufficient energy (hv), the excess kinetic energy (KE) of the emitted electrons being related to the ionization energy (IE) through the equation... 

---