Electron photon scattering

Arthur Holly Compton (1892-1962) was an American physicist and professor at the universities of Saint Louis and Chicago. He obtained the Nobel Prize in 1927 for the discovery of the effect named after him i.e.. for investigations of electron-photon scattering. 

There are situations in which a definite wave function cannot be ascribed to a photon and hence cannot quantum-mechanically be described completely. One example is a photon that has previously been scattered by an electron. A wave function exists only for the combined electron-photon system whose expansion in terms of the free photon wave functions contains the electron wave functions. The simplest case is where the photon has a definite momentum, i.e. there exists a wave function, but the polarization state cannot be specified definitely, since the coefficients depend on parameters characterizing the other system. Such a photon state is referred to as a state of partial polarization. It can be described in terms of a density matrix... 

The interaction of even simple diatomic molecules with strong laser fields is considerably more complicated than the interaction with atoms. In atoms, nearly all of the observed phenomena can be explained with a simple three-step model [1], at least in the tunneling regime (1) The laser field releases the least bound electron through tunneling ionization (2) the free electron evolves in the laser field and (3) under certain conditions, the electron can return to the vicinity of the ion core, and either collisionally ionize a second electron [2], scatter off the core and gain additional kinetic energy [3], or recombine with the core and produce a harmonic photon [4]. 

Bulk silicon is a semiconductor with an indirect band structure, as schematically shown in Fig. 7.12 c. The top of the VB is located at the center of the Brillouin zone, while the CB has six minima at the equivalent (100) directions. The only allowed optical transition is a vertical transition of a photon with a subsequent electron-phonon scattering process which is needed to conserve the crystal momentum, as indicated by arrows in Fig. 7.12 c. The relevant phonon modes include transverse optical phonons (TO 56 meV), longitudinal optical phonons (LO 53.5 meV) and transverse acoustic phonons (TA 18.7 meV). At very low temperature a splitting (2.5 meV) of the main free exciton line in TO and LO replicas can be observed [Kol5]. 

Inelastic photon scattering processes are also possible. In 1928, the Indian scientist C. V. Raman (who won the Nobel Prize in 1930) demonstrated a type of inelastic scattering that had already been predicted by A. Smekal in 1923. This type of scattering gave rise to a new type of spectroscopy, Raman spectroscopy, in which the light is inelastically scattered by a substance. This effect is in some ways similar to the Compton effect, which occurs as a result of the inelastic scattering of electromagnetic radiation by free electrons. 

If a single crystal is rotated in a monochromatic X-ray beam, a pattern of spots of reinforced X-rays can be recorded, traditionally on a photographic film placed behind the crystal perpendicular to the primary beam (giving the so-called Laue photographs). Nowadays, X-ray diffractometers use electronic photon counters as detectors. Since, as noted above, different atoms have different X-ray scattering powers, both the positions and... 

In UHV surface spectroscopies, the electrode under investigation is bombarded by electrons, photons, or ions, and an analysis of the electrons, ions, molecules, or atoms scattered or released from the surface provides information related to the electronic and structural parameters of the atoms and ions in the interfacial region. As mentioned before, the transfer of the electrode from the electrochemical cell to the UHV chamber is a crucial step in the use of these techniques. This has motivated a few groups to build specially designed transfer systems. Pioneering work in this area was done by Hubbard s group, followed by Yeager. 

In the variety of excitation or de-excitation processes that allow the preparation and/or observation of the system via the participation of the continuous spectrum, the dominant and most interesting characteristics are generated by the transient formation of nonstationary or unstable states. For example, the excitation may be caused by the absorption of one or of many photons during the interaction of an initial atomic or molecular state with pulses of long or of short duration. Or, the transient formation and influence on the observable quantity may occur during the course of electron-atom scattering or of chemical reactions. 

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. 

Nanocrystals and nanowires are utilized in a new generation of solar collectors (a nanometer is one billionth of a meter). In conventional solar cells, at the P-N junction one photon splits one electron from its "hole companion" as it travels to the electron-capturing electrode. If solar collectors are made of semiconducting nanocrystals that disperse the light, according to TU Delft s professor Laurens Siebbeles, an avalanche effect results and one photon can release two or three electrons, because this effect maximizes photon absorption while minimizing electron-hole recombination. This effect of the photon-scattering nanoparticles substantially increases cell efficiency. 

Fig. 1. Time-direct - (a) - and time-reverse - (b) - diagrams describing the process of inelastic scattering of a photon with the energy hu> and the wave vector k by an atom residing in the state ) (ui,k, ) —> (u>, k, f)). Solid lines stand for atomic states, dashed lines denote photons in the initial/final states, and filled circles designate the vertices of the electron-photon interaction V...

Figure 1 depicts a typical sequence of events started by absorption of an incident photon with an energy near the nuclear excited state energy Eq. The Fe nucleus has an excited state lifetime of 141ns, and excited nuclei have two decay channels. About 10% of them reemit a 14.4kev photon. For recoilless absorption, where no vibrational levels are excited, time-resolved measurements of 14.4kev photons scattered in the forward direction reveal information on hyperfine interactions comparable to conventional Mossbauer spectroscopy (see Mossbauer Spectroscopy). The remaining nuclei expel electrons from the atomic K shell, followed by... 

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