Phonon processes, many

When lowering the temperature, the many phonon processes die out and the infrared absorption gets narrower. Correlation effects between the randomized charges in the disordered material tend to diminish the low-frequency tail further [ ]. Instead of this, the spectrum at 4.2 K shows two features which are absent at... 

The observations of complex dynamics associated with electron-stimulated desorption or desorption driven by resonant excitation to repulsive electronic states are not unexpected. Their similarity to the dynamics observed in the visible and near-infrared LID illustrate the need for a closer investigation of the physical relaxation mechanisms of low energy electron/hole pairs in metals. When the time frame for reaction has been compressed to that of the 10 s laser pulse, many thermal processes will not effectively compete with the effects of transient low energy electrons or nonthermal phonons. It is these relaxation channels which might both play an important role in the physical or chemical processes driven by laser irradiation of surfaces, and provide dramatic insight into subtle details of molecule-surface dynamics. 

Quenching of narrow-line emissions (as observed for many Ln3+ ions) has been explained by phonon emission to the lattice modes. Moos and co-workers (60) and others (67) have given many examples. Usually the nonradiative rate is described by Kiel s formula (62) for a single-frequency p-phonon process,... 

Many-phonon hopping processes could be treated as defect localization during some waiting time to with further sudden and instant hop and new localization in a nearest lattice site for tq etc. The same situation takes place for solvated electrons in solutions and electron migration over impurities in... 

The pseudospin methodology is widely used not only for the description of hydrogen containing ferro- and antiferroelectrics, but also for the study of many other systems with hydrogen bonds. In particular, the pseudospin methodology was applied by Silbey and Trommsdorff [99] for examining the influence of two-phonon process on the rate constant of molecular compounds. In the next sections we will also employ the pseudospin formalism for the investigation of some problems where protons are exemplified by the cooperative behavior. 

These cations are essentially acceptors, even for resonant energy (dq) transfer. Since they cannot undergo an excitation transition because of ground state coupling to the local phonon modes, they function conversely to the luminescence process. Many of them exhibit strong absorption bands at the frequencies of light commonly used for excitation of phosphors. Therefore, excitation energy is dissipated to the lattice by phonon processes, once it has been captured by this type of site. 

In a two-phonon process which involves an internal mode of vibration for which the frequency p varies little with wave vector, the combined density of states approximates, at least in a cubic crystal, the density of vibrational states spectrum [see Equation (6)]. This is important for a discussion of the absorption that occurs on the high- and low-frequency sides of many internal-mode absorption lines. 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

However, most impurities and defects are Jalm-Teller unstable at high-symmetry sites or/and react covalently with the host crystal much more strongly than interstitial copper. The latter is obviously the case for substitutional impurities, but also for interstitials such as O (which sits at a relaxed, puckered bond-centred site in Si), H (which bridges a host atom-host atom bond in many semiconductors) or the self-interstitial (which often fonns more exotic stmctures such as the split-(l lO) configuration). Such point defects migrate by breaking and re-fonning bonds with their host, and phonons play an important role in such processes. 

An interesting aspect of many structural phase transitions is the coupling of the primary order parameter to a secondary order parameter. In transitions of molecular crystals, the order parameter is coupled with reorientational or libration modes. In Jahn-Teller as well as ferroelastic transitions, an optical phonon or an electronic excitation is coupled with strain (acoustic phonon). In antiferrodistortive transitions, a zone-boundary phonon (primary order parameter) can induce spontaneous polarization (secondary order parameter). Magnetic resonance and vibrational spectroscopic methods provide valuable information on static as well as dynamic processes occurring during a transition (Owens et ai, 1979 Iqbal Owens, 1984 Rao, 1993). Complementary information is provided by diffraction methods. 

The aim of this chapter is to clarify the conditions for which chemical kinetics can be correctly applied to the description of solid state processes. Kinetics describes the evolution in time of a non-equilibrium many-particle system towards equilibrium (or steady state) in terms of macroscopic parameters. Dynamics, on the other hand, describes the local motion of the individual particles of this ensemble. This motion can be uncorrelated (single particle vibration, jump) or it can be correlated (e.g., through non-localized phonons). Local motions, as described by dynamics, are necessary prerequisites for the thermally activated jumps responsible for the movements over macroscopic distances which we ultimately categorize as transport and solid state reaction.. 

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