Photoelectron emitted

The experimental work described in this chapter clearly demonstrates that chiral asymmetries in the forward-backward distribution of photoelectrons emitted from randomly oriented enantiomers when ionized with circularly polarized light can be spectacularly large (to borrow and apply a superlative from previous accounts of an unprecedented chiral asymmetry)—on the order of 20%. The theory discussed here, as implemented in two computational methods, is fully capable of predicting this and being applied to develop an understanding of a phenomenon that at times displays some counterintuitive properties. Doing so is very much an ongoing quest. 

Figure 4.6. Photoemission and the Auger process. Left An incident X-ray photon is absorbed and a photoelectron emitted. Measurement of its kinetic energy allows one to calculate the binding energy of the photoelectron. The atom becomes an unstable ion with a hole in one of the core levels. Right The excited ion relaxes by filling the core hole...

Lifetime instruments using a streak camera as a detector provide a better time resolution than those based on the single-photon timing technique. However, streak cameras are quite expensive. In a streak camera, the photoelectrons emitted... 

Even in the absence of illumination (darkness) some electrons, excited by thermal energy, are emitted from the photocathode. Since photocathodes are materials with low working functions, the thermal energy can be high enough to induce the emission of electrons. These emitted electrons give rise to what is known as the dark current or, sometimes, the thermo-ionic current. The dark current varies randomly with time, so that it is considered as noise. It has been experimentally determined that the thermo-ionic current, U, due to photoelectrons emitted by a photocathode in the absence of illumination is given by... 

X ray photoelectron spectroscopy (XPS) is powerful in identifying species present at the surface/interface and atoms or functional groups involved in acid-base interactions [116]. Since XPS measures the kinetic energy of photoelectrons emitted from the core levels of surface atoms upon X ray irradiation of the uppermost atomic layers, it can be used to characterize surface acid sites, in combination with base probe molecules adsorption. 

X-Ray Photoelectron Spectrometry. X-ray photoelectron spectrometry (XPS) was applied to analyses of the surface composition of polymer-stabilized metal nanoparticles, which was mentioned in the previous section. This is true in the case of bimetallic nanoparticles as well. In addition, the XPS data can support the structural analyses proposed by EXAFS, which often have considerably wide errors. Quantitative XPS data analyses can be carried out by using an intensity factor of each element. Since the photoelectron emitted by x-ray irradiation is measured in XPS, elements located near the surface can preferentially be detected. The quantitative analysis data of PVP-stabilized bimetallic nanoparticles at a 1/1 (mol/mol) ratio are collected in Table 9.1.1. For example, the composition of Pd and Pt near the surface of PVP-stabilized Pd/Pt bimetallic nanoparticles is calculated to be Pd/Pt = 2.06/1 (mol/ mol) by XPS as shown in Table 9.1.1, while the metal composition charged for the preparation is 1/1. Thus, Pd is preferentially detected, suggesting the Pd-shell structure. This result supports the Pt-core/Pd-shell structure. The similar consideration results in the Au-core/Pd-shell and Au-core/Pt-shell structure for PVP-stabilized Au/Pd and Au/Pt bimetallic nanoparticles, respectively (53). 

Two rather different techniques that exploit the same underlying phenomenon of coherent interference of elastically scattered low energy electrons are photoelectron diffraction [5] and surface extended X-ray absorption fine structure (SEXAFS) [6,7]. Figure 1.1. shows schematically a comparison of the electron interference paths in LEED and in these two techniques. In both photoelectron diffraction and SEXAFS the source of electrons is not an electron beam from outside the surface, as in LEED, but photoelectrons emitted from a core level of an atom within the adsorbate. In photoelectron diffraction one detects the photoelectrons directly, outside the surface, as a function of direction or photoelectron energy (or both). The detected angle-resolved photoemission signal comprises a coherent sum of the directly emitted component of the outgoing photoelectron wavefield and other components of the same wavefield elastically scattered by atoms (especially in the substrate) close... 

Fig. 17. Kinetic energy distribution of photoelectrons emitted from a chlorophyll a...

Photomultipliers are vacuum tube photocells with a sealed-in set of dynodes. Each successive dynode is kept at a potential difference of 100V o that photoelectrons emitted from the cathode surface are accelerated M each step. The secondary electrons ejected from the last dynode are Collected by the anode and are multiplied so that a 10° — 107 — fold arnpli-t tfion of electron flux is achieved. This allows simple devices such as l croammeters to measure weak light intensities. Background thermal mission can be minimised by cooling the photomultiplier. The schematic... 

Fig. 8 Normalized temperature dependent photoelectron energy distribution spectra for photoelectrons emitted from gold coated with monolayers of (a) LC polyalanine, and (b) DN polyalanine. The laser wavelength was 266 nm (4.66 eV)...

A method that yields local structural information is EXAFS which utilizes the scattering of photoelectrons emitted from specific atomic core levels for determining the interatomic distances and counting the near neighbors (but does not determine the directions between the emitting atom and its neighbors). 

Though a typical XPS detector collects all emitted photons, regardless of their ejection angles, it should be noted that angle-resolved XPS (ARPES) and UPS (ARUPS) may also be carried out. By detecting photoelectrons emitted from a surface at different emission angles, one obtains the energy of the electrons as a function... 

The photoelectron intensity for a given element A is determined by the product of this element s concentration level and its effective photo-ionisation cross section (for the orbital under consideration) For this element, the number of photoelectrons emitted is thus proportional to A fraction T of these electrons is effectively transmitted to the... 

Obviously quantitative information should also be available from XPS spectra because the flux of photoelectrons emitted is proportional to the number of emitting atoms. The relationship between the peak intensities in spectra similar to the survey spectrum shown in Figure 4, and the surface concentrations of the elements detected is however not simple. The basic reason for this is that the photoelectron flux is attenuated on its path through the solid following a Beer-Lambert law ... 

FIGURE 4.15 Frequency and intensity dependence of the photoelectric effect. Only light above the threshold frequency can eject photoelectrons from the surface. Once the frequency threshold has been passed, the total current of photoelectrons emitted depends on the intensity of the light, not on its frequency. 

XPEEM is a spectromicroscopy technique utilizing near edge X-ray absorption fine structure (NEXAFS) contrast. XPEEM probes the surface chemistry by detection of photoelectrons emitted from the surface. XPEEM was performed at the PEEM2 microscope at the bending magnet beamline... 

X-ray Photoelectron Spectroscopy (XPS). In XPS, the sample is bombarded with soft x-rays and the photoelectrons emitted are analyzed in terms of their kinetic energy. Eg. The resulting core level peaks such as the C Is are due to photoelectrons emitted from the atomic (core) orbitals of the atoms in the surface layers present. The binding energies, Eg, of these electrons were obtained from ... 

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