Analysers electron energy

Figure Bl.6.3 Electron energy analysers that use magnetic fields (a) the trochoidal analyser employing an electromagnet, (b) the Wien filter and (c) the sector magnet analyser. Trajectories for electrons of different energies are shown.

Xps ndAes Instrumentation. The instmmentation required to perform xps and aes analyses is generally sophisticated and expensive (19). The need for UHV conditions in order to retain surface cleanliness for a tractable period of time was mentioned above. Beyond this requirement (and the hardware that accompanies it), the most important components of an electron spectrometer system are the source, the electron energy analyzer, and the electron detector. These will be discussed in turn below. 

Figure 7 Schematic of a typieai eiectron spectrometer showing aii the necessary components. A hemisphericai electrostatic electron energy analyser is depicted.

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

There are two operating modes known as Constant Retard Ratio (CRR) or Constant Analysis Energy (CAE). In CRR, the electrons are slowed down by an amount which is a constant ratio of the electron energy to be analysed. For example, if the retard ratio is 10 and 1000 eV electrons are to be detected, then the electrons will be slowed down to 100 eV and the pass energy will be set to 100 eV. In CAE, the pass energy is fixed. If the pass energy is 50 eV, then electrons of 1000 eV will have to be slowed down by 950 eV in order to be detected. 

Figure 2.3. Schematic diagram of a concentric hemispherical electron energy analyser.

Excitation of sample by bombardment with electrons, radioactive particles or white X-rays. Dispersive crystal analysers dispersing radiation at angles dependent upon energy (wavelength), detection of radiation with gas ionization or scintillation counters. Non-dispersive semiconductor detectors used in conjunction with multichannel pulse height analysers. Electron beam excitation together with scanning electron microscopes. 

Fig. 10. Photoeleotron spectrum of oxygen using the helium resonance line (21-21 e.v.) obtained with a magnetic electron energy analyser (May and Turner, unpublished work). Ionization energy increasing from left to right. The spectrum reveals four levels of ionization and the vibrational structure associated with each state of the ion can be clearly distinguished. This spectrum may be compared with that obtained using an electrostatic retarding field analyser (Al-Joboury et al., 1965).

In the case of BIS, monoenergetic electrons from an electron gun impinge on the sample the reponse to be analysed is constituted of photons. No heavy and sophisticated electron energy analyser is necessary. Therefore, protection against radioactive hazards can be easily achieved by inserting the complete spectrometer into a glove box system. 

I drew attention in Chapter 12 to the fact that the Xa orbitals did not satisfy the nice properties of standard HF-LCAO ones the Koopmans theorem is not valid, and so on. The same is true of all density functional KS-LCAO calculations. In practice, it usually turns out that the KS-LCAO orbitals are very similar to ordinary HF-LCAO ones, which must mirror the fact that exchange-correlation effects are only a minor part of the total electronic energy. So the orbitals are often analysed as if they were ordinary HF orbitals (Figure 13.4). 

Fig. 1.11 Photoelectron spectroscopy, (a) Schematic illustration of apparatus, comprising radiation source, sample, electron energy analyser and detector, all in a vacuum chamber, (b) Spectrum obtained from solid CdO, using X-rays of photon energy 1284 eV. (c) Interpretation of peaks in spectrum. The zero of energy in this scale corresponds to electrons with just sufficient energy to leave the solid positive values are the kinetic energies of emitted electrons, negative values correspond to the binding energies of electrons in the solid.

Whereas HF theory is good for isolated, gas-phase molecules, some potential problems should be noted. One is the failure of a calculation to reach an SCF. There is no guarantee every SCF run will converge to a stable solution, without which all subsequent analyses of energies, charge distribution, and other molecular properties are precluded. Most calculations run okay, but sometimes electron density will oscillate between the two sides of a molecule, thereby preventing the achievement of an SCF. Techniques are available in many MO programs to damp down on the oscillation if it arises and to otherwise help reach a satisfactory endpoint, i.e., the SCF... 

The present work is firstly focused on the Dih Rh2+ centre in NaCl where active electrons are lying just in the RhClg- complex formed with six closest anions [10]. By means of adiabatic DFT calculations performed for the perfect Oh geometry and also for different values of the Qe coordinate the meaning and weight of parameters involved in the model-Hamiltonian are analysed. To this aim particular attention is paid to the Qe dependence of one electron energies, sa and sb, associated with the antibonding alg ( 3z2 — r2) and blg ( x2 — y2) orbitals. 

In Part A the aims and the potential of electron spectrometry of free atoms have been discussed for the example of photoionization and subsequent Auger decay in neon. Now the apparatus details and the basic features of the technique of electron spectrometry will be considered. The discussion is restricted to electrostatic deflection analysers. However, the properties discussed can easily be adapted and transferred to other kinds of electron energy analysers. 

In order to analyse electrons with respect to their kinetic energy, an energy dispersive element is needed which separates the electrons according to their energy. For this purpose one of the following properties of moving electrons can be used (non-relativistic electrons only are considered) ... 

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