Crystal interaction with photon

Phonons interact with photons, electrons and neutrons, causing scattering. This causes a beam of radiation incident on a crystal to spread out, and a diffracted beam will be broadened by this extra contribution. The extent of the spreading is related to the phonon spectrum in the crystal, and its measurement gives information on the phonon distribution in the solid. 

The probability for photoelectric absorption drops below that for Compton scattering at about 150 keV in germanium. That large crystals can be used for energies of many MeV is due to multiple interactions with photons and a sufficient size to absorb the energetic secondary electrons. At energies well above the 1,022 keV threshold, production of electron—positron... 

Figure 1 Schematic of an EDS system on an electron column. The incident electron interacts with the specimen with the emission of X rays. These X rays pass through the window protecting the Si (Li) and are absorbed by the detector crystal. The X-ray energy is transferred to the Si (Li) and processed into a dig-itai signal that is displayed as a histogram of number of photons versus energy.

SGX-CAT maintains a direct T1 network connection from the Advanced Photon Source in Illinois to SGX San Diego. Database inquiries are handled over this link. Interactions with the database occur in three ways. An extensive web-based system is used for data entry and retrieval. For crystals generated by external users of the beamline, upload of an electronic spreadsheet transfers the required crystal data attributes to the SGX LIMS. For automated operations, such as crystal screening and data collection, custom scripts place the computed results directly into the database. 

Compared with the momentum of impinging atoms or ions, we may safely neglect the momentum transferred by the absorbed photons and thus we can neglect direct knock-on effects in photochemistry. The strong interaction between photons and the electronic system of the crystal leads to an excitation of the electrons by photon absorption as the primary effect. This excitation causes either the formation of a localized exciton or an (e +h ) defect pair. Non-localized electron defects can be described by planar waves which may be scattered, trapped, etc. Their behavior has been explained with the electron theory of solids [A.H. Wilson (1953)]. Electrons which are trapped by their interaction with impurities or which are self-trapped by interaction with phonons may be localized for a long time (in terms of the reciprocal Debye frequency) before they leave their potential minimum in a hopping type of process activated by thermal fluctuations. 

Most of the irregular SE s formed by irradiation interact with impurities that are the native irregular SE s of the crystal. Impurities interact with the irradiation products either by their stress field or, if heterovalent, by the electrostatic (Coulomb) field. Photolysis (radiolysis) is found in other than halide crystals as well. In oxides, the production of Frenkel pairs under photon irradiation is negligible. This has been ascribed to the fact that the reaction O2- +0" = 02 is endothermic, whereas the reaction X- +X = is exothermic. 

In the first part of this introductory section, we summarize the main collective phenomena acquired by the dipolar exciton from the lattice-symmetry collectivization of molecular properties. The crystal is considered as an assembly of electrically neutral systems, the molecules, physically separated from each other and in electromagnetic interaction. This /V-body problem will be treated quantum-mechanically in the limit of low exciton densities. We redemonstrate the complete equivalence of this treatment with the theories of Lorentz and Ewald, as well as with the semiclassical approximation. In Section I.A, in a more compact but still gradual way, we establish the model of the rigid lattice of dipoles and the general theory of low-exciton-density systems in interaction with the radiation field. Coulombic excitons, photons,... 

Surface-enhanced exciton dissociation models are based on the assumption that the absorption of a photon creates an exciton that diffuses to the surface where it dissociates into a free electron-hole pair, or a free and deeply trapped carrier of opposite sign, through an interaction with a donor or acceptor center associated with the surface. Such a process was first proposed by Lyons (1955). Evidence for this was largely based on early studies of anthracene, where the photogeneration efficiency increases with the absorption coefficient. Further evidence was that gases, particularly O2, adsorbed on the surface of anthracene crystals significantly change the photoconductivity, even for weakly absorbed radiation. 

Yet another physical phenomenon is used in solid-state detectors, which are manufactured from high quality silicon or germanium single crystals doped with lithium and commonly known as Si(Li) or Ge(Li) solid-state detectors. The interaction of the x-ray photon with the crystal (detector) produces electron-hole pairs in quantities proportional to the energy of the photon divided by the energy needed to generate a single pair. The latter is... 

The photon-phonon transformation, obtained when a photon interacts with a crystal and gives information on the vibrational levels of the crystal lattice, cannot be applied to the laser induced... 

One of the simplest methods to create excitons is to use electromagnetic radiation. Below a physical picture will be used which existed before the polariton concept had been formulated (see Ch. 4). In this picture the crystal photon with wavevector q and energy hui = hqc propagates in the crystal and can interact with crystal electrons and can decay, creating an exciton. Due to energy and momentum conservation, the energy of an exciton created by the photon... 

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