77-electron energy

It was stated in Chapter 2 that organic molecules in the vapor state are ionized when an amount of energy equal to the ionization potential is transferred from an electron beam to the molecules, as depicted in Equation 4.1  

The ionization potential I can be measured by mass spectrometry. The minimum electron energy at which the molecular ion is formed is determined by lowering the potential between the filament and electron trap in small steps until the molecular ion is no longer detected. 

If the energy transferred to the molecules is much greater than the ionization potential, fragmentation occurs. There is a minimum energy that must be supplied to form each fragment ion from a molecule. This is termed the appearance potential A of the ion and can be measured in the same manner, as depicted in Equation 4.2  

For the molecular ion XY+, the energy required to break the X-Y bond (Dx.y) can be calculated from the ionization and appearance potentials it is simply the difference between the two (Equation 4.3)  

Using an electron beam with low energy (10-30 eV) increases the intensity of a molecular ion relative to the fragment ion intensities. This technique offers evidence for whether a particular observed ion is in fact a molecular ion or a fragment. 

---

This energy region is that which corresponds to the electron energy states of the molecules, which are quantified and so can have only well-defined discrete values. 

Fig. VIII-10. (a) Intensity versus energy of scattered electron (inset shows LEED pattern) for a Rh(lll) surface covered with a monolayer of ethylidyne (CCH3), the structure of chemisorbed ethylene, (b) Auger electron spectrum, (c) High-resolution electron energy loss spectrum. [Reprinted with permission from G. A. Somoijai and B. E. Bent, Prog. Colloid Polym. ScL, 70, 38 (1985) (Ref. 6). Copyright 1985, Pergamon Press.]...

EELS Electron-energy-loss Incident electrons are Surface energy states ... 

HREELS High-resolution electron energy-loss spectroscopy [129, 130] Same as EELS Identification of adsorbed species through their vibrational energy spectrum... 

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... 

APS Appearance potential spectroscopy (see AES) Intensity of emitted x-ray or Auger electrons is measured as a function of incident electron energy Surface composition... 

H. Ibach and D. L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic, New York, 1982. 

If one uses a Slater detemiinant to evaluate the total electronic energy and maintains the orbital nomialization, then the orbitals can be obtained from the following Hartree-Fock equations ... 

Using the above expression and equation Al.3.19. the total electron energy, for a free electron gas... 

The electronic energy, as detennined from must be added to tire ion-ion interactions to obtain the structural energies. This is a straightforward calculation for confined systems. For extended systems such as crystals, the calculations can be done using Madelimg summation techniques [2]. 

Figure Al.3.23. Phase diagram of silicon in various polymorphs from an ab initio pseudopotential calculation [34], The volume is nonnalized to the experimental volume. The binding energy is the total electronic energy of the valence electrons. The slope of the dashed curve gives the pressure to transfomi silicon in the diamond structure to the p-Sn structure. Otlier polymorphs listed include face-centred cubic (fee), body-centred cubic (bee), simple hexagonal (sh), simple cubic (sc) and hexagonal close-packed (licp) structures.

The resolution of this issue is based on the application of the Pauli exclusion principle and Femii-Dirac statistics. From the free electron model, the total electronic energy, U, can be written as... 

The rotation-vibration-electronic energy levels of the PH3 molecule (neglecting nuclear spin) can be labelled with the irreducible representation labels of the group The character table of this group is given in table Al.4.10. 

At a surface, not only can the atomic structure differ from the bulk, but electronic energy levels are present that do not exist in the bulk band structure. These are referred to as surface states . If the states are occupied, they can easily be measured with photoelectron spectroscopy (described in section A 1.7.5.1 and section Bl.25.2). If the states are unoccupied, a teclmique such as inverse photoemission or x-ray absorption is required [22, 23]. Also, note that STM has been used to measure surface states by monitoring the tunnelling current as a fiinction of the bias voltage [24] (see section BT20). This is sometimes called scamiing tuimelling spectroscopy (STS). 

Electrons interact with solid surfaces by elastic and inelastic scattering, and these interactions are employed in electron spectroscopy. For example, electrons that elastically scatter will diffract from a single-crystal lattice. The diffraction pattern can be used as a means of stnictural detenuination, as in FEED. Electrons scatter inelastically by inducing electronic and vibrational excitations in the surface region. These losses fonu the basis of electron energy loss spectroscopy (EELS). An incident electron can also knock out an iimer-shell, or core, electron from an atom in the solid that will, in turn, initiate an Auger process. Electrons can also be used to induce stimulated desorption, as described in section Al.7.5.6. 

PES of neutral molecules to give positive ions is a much older field [ ]. The infomiation is valuable to chemists because it tells one about unoccupied orbitals m the neutral that may become occupied in chemical reactions. Since UV light is needed to ionize neutrals, UV lamps and syncln-otron radiation have been used as well as UV laser light. With suitable electron-energy resolution, vibrational states of the positive ions can be... 

In this chapter we shall first outline the basic concepts of the various mechanisms for energy redistribution, followed by a very brief overview of collisional intennoleciilar energy transfer in chemical reaction systems. The main part of this chapter deals with true intramolecular energy transfer in polyatomic molecules, which is a topic of particular current importance. Stress is placed on basic ideas and concepts. It is not the aim of this chapter to review in detail the vast literature on this topic we refer to some of the key reviews and books [U, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32] and the literature cited therein. These cover a variety of aspects of tire topic and fiirther, more detailed references will be given tliroiighoiit this review. We should mention here the energy transfer processes, which are of fiindamental importance but are beyond the scope of this review, such as electronic energy transfer by mechanisms of the Forster type [33, 34] and related processes. 

Resonant processes of some importance include resonant electronic to electronic energy transfer (E-E), such as the pumping process of the iodine atom laser... 

Vibrational spectroscopy provides detailed infonnation on both structure and dynamics of molecular species. Infrared (IR) and Raman spectroscopy are the most connnonly used methods, and will be covered in detail in this chapter. There exist other methods to obtain vibrational spectra, but those are somewhat more specialized and used less often. They are discussed in other chapters, and include inelastic neutron scattering (INS), helium atom scattering, electron energy loss spectroscopy (EELS), photoelectron spectroscopy, among others. 

The teclmologies of die various electron spectroscopies are similar in many ways. The teclmiques for measuring electron energies and the devices used to detect electrons are the same. All electron spectrometers... 

---