Scatter inelastically

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. 

In a closer view (Eig. 2.32), the following interactions occur on an atomic scale when a material is hit by electrons. Eirstly, in addition to elastic scattering, inelastic scatter-... 

In Fig. 3, we placed the cut line between ([) = e and ([) = 2ti e, where e is an arbitrarily small number. This choice will often be the most convenient cut line, because n then exactly describes the number of complete loops that the system has made around the Cl since entering the encirclement region. Thus the paths that scatter inelastically will each have described an (internal) rotation of exactly < ) = 2nn. 

A substantial number of electrons are elastically scattered, and this gives rise to a strong elastic peak in the spectrum. When an electron of low energy (2-5 eY) approaches a surface, it can be scattered inelastically by two basic mechanisms, and the data obtained are dependent upon the experimental geometry - specifically the angles of the incident and the (analysed) scattered beams with respect to the surface (0 and 02 in Figure 5.47). Within a certain distance of the surface the incident electron can interact with the dipole field associated a particular surface vibration, e.g. either the vibrations of the surface atoms of the substrate itself, or one or other... 

When an energetic electron scatters inelastically, an electron from the (filled) valence band can be promoted to the (empty) conduction band creating an electron/hole pair. On recombination, the excess energy is released as a photon, the wavelength of which is well defined by the band-gap transition. The technique is powerful in catalysis it is diagnostic of the electronic/chemical state and is sensitive to point defects. It can be used to probe the distribution of dopants in catalytic oxides. 

Light that is scattered from a molecule is primarily elastically scattered that is, the incident and the scattered photons have the same energy. A small probability exists, however, that a photon is scattered inelastically, resulting in either a net gain or loss of energy of the scattered photon. This inelastic scattering, discovered by Raman and Krishna,1 allows fundamental molecular vibrational transitions to be measured at any excitation wavelength. 

The commonly used scheme of energy relaxation in RGS includes some stages (Fig.2d, solid arrows). Primary excitation by VUV photons or low energy electrons creates electron-hole pairs. Secondary electrons are scattered inelastically and create free excitons, which are self-trapped into atomic or molecular type centers due to strong exciton-phonon interaction. 

In the collision-induced Raman experiments the laser light of wavenumber co0 is scattered inelastically by the interacting atoms in the gas. The intensities of the depolarized, D(v), and polarized, P(v), scattered light are given by323 324,... 

Figure 14.3 Experimental Cu KL23L23 Auger spectrum (dots) photoexcited from a approximately 100-nm thick polycrystalline layer using Mo bremsstrahlung [11]. The solid and dashed lines indicate the contributions from electrons scattered inelastically within the solid sample, estimated by different models [11],...

Figure 9-4. The Raman and resonance Raman scattering processes. In this figure, i > and f > refer to the initial and final states, respectively, in the Raman or resonance Raman scattering process. (A) In the Raman process, the molecule is initally in its ground vibrational level of the ground electronic state. An excitation photon (up arrow) carries the molecule to a virtual level (dashed line) from which it immediately scatters inelastically (down arrow), leaving die molecule in an excited vibrational level of the ground state. The difference between die excitation and scattered photon is measured. (B) Resonance Raman scattering follows the same process, except that die virtual level (dashed line) is coincident with a real excited vibronic level of the molecule. Again, die difference between the excitation and scattered photon is measured...

Thus, in the Stokes case, the molecule is initially in its lowest-lying vibrational levels. The incident photon, at an energy much lower than necessary to reach the lowest-lying excited electronic state (i.e. the sample is transparent at this wavelength), excites the molecule to a virtual state (dashed line in Figure 9-4A) from which it immediately scatters inelastically. The scattered photon is at lower energy than the exciting photon and the molecule is vibrationally excited in one or more vibrational modes. 

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