Excitons mechanical

The photoreaction of oxidation of water was discovered in 1927 by Baur and Neuweiler (76) and investigated later by a number of workers. The analysis of experimental results performed by Korsunovsky (65-68) is based on the exciton mechanism of light absorption. The kinetics of the reaction has been investigated by Grossweiner (77). 

In conclusion we stress once more that the above-considered mechanism of the effect of illumination on the adsorptivity and catalytic activity of a semiconductor holds in the case when the absorption of light increases the number of free electrons or holes (or both) in the crystal. This, however, does not always take place. The absorption of light by the crystal may proceed by an exciton mechanism. This seems to be the case in the region of intrinsic absorption, which is as a rule photoelectrically inactive. 

For the exciton mechanism of defect production in alkali halides the Frenkel pairs of well correlated defects are known to be created [35], the mean distance between defects inside these pairs is much smaller than that between different pairs. The geminate pair distribution function could often be approximated as... 

The migration of triplet energy in pure or mixed crystals by an exciton mechanism has been well established very recently.86 Thus the occurrence of delayed fluorescence in organic crystals is explained by triplet-triplet annihilation following triplet exciton migration.86 However, as yet, no conclusive evidence for the occurrence of triplet exciton migration in fluid media has been forthcoming. 

In contrast to the trans-disubstituted phenylcyclopropanes cis-disubstituted phenylcyclopropanes, such as 23 and 30, do not show any detectable CD signals a. This is reasonable, if the optical activity of the two lowest energy electronic bands of phenylcyclopropanes are generated essentially by the coupled-oscillator mechanism (exciton mechanism for the degenerate case, such as 19). In cis-phenylcyclopropanes the interacting transition moments will lie (approximately) in the same plane. Hence, in the coupled-oscillator mechanism there will be only a very small Cotton effect (which is zero, if... 

Here E is the frequency of the pairing-mediating excitation, N(Ef) the density of states at the Fermi level, and V the electron-excitation coupling strength. In view of the high transition temperature of 28 K and the polarizable jv system of the Ceo molecule, excitonic mechanisms of the type envisaged by Little, where E is an electronic excitation energy, may be of relevance. 

The CD of the CT transitions has attracted little direct interest in the literature. Mason (10) has postulated that it arises from an exciton mechanism in which the CT transition is broken up into three degenerate oscillators or chromophores with different origins. Such a procedure is unjustified quantum mechanically, as each chromophore must have negligible electron exchange with any other chromophore considering that a common metal d state is involved in the definition of each such chromophore or oscillator, such a model is, despite its pictorial appeal, theoretically inconsistent. Mason supports his model by noting that the CT CD exhibits the exciton structure characteristic of... 

One possible mechanism which must be considered is the excitonic mechanism. We suggested that mixed valence, layered perovskite related structured materials would be best suited as... 

It is suggested that pressure enhances the screening of electron-electron repulsion via the excitonic mechanism. 

Many authors have discussed the excitonic mechanism (36-37.) of superconductivity, in which the effective attractive interaction between conduction electrons originates from virtual excitations of excltons rather than phonons. The basic idea of the models proposed is that conduction electrons residing on the conducting filament (or plane) induce electronic transitions on nearby easily polarizable molecules (or complexes), which result in an effective attractive interaction between conduction electrons. As perhaps a striking realization of the excitonic mechanism of superconductivity,... 

Tsai, T. E. and Griscom, D. L., Experimental evidence for excitonic mechanism of defect generation in high-purity silica, Phys. Rev. Lett. 67, 2517 (1991). 

The present model for nerve impulse resembles closely the exciton mechanism of high-temperature superconductivity, as put forward by Little and Ginzburg. Little s polymer consists of a polyene spine with polarizable dye side chains, the latter forming the exciton band. Ginzburg proposes high-temperature superconductivity to be found in thin metallic films placed between highly dielectric layers. " ... 

This trend goes further away from the peculiar physical properties which were predicted for 1-D electronic systems. There are the Peierls-Frdhlich CDW collective transport mechanism [30], and Little s excitonic mechanism for high-T superconductivity [33], in which an electronically polarizable entity is used instead of the ionic lattice. It is necessary to conclude that there is a divorce between the experimental results and these initial attractive concepts. 

Here the dimensionless interaction parameter Xaf = (/( f>o)/( l ) describes the strength of the AF influence on A(k). It is interesting to note that despite the presence of a node line the above order parameter is fully symmetric (Aig) because it is invariant under all symmetry transformations of the magnetic unit cell. Thus in a strict sense UPd2Al3 cannot be called a superconductor with unconventional gap symmetry, although the novel magnetic exciton mechanism certainly is unconventional. Various possible nodal stractures in addition to eq. (90) have been discussed by Thalmeier and MaM (2002) in the context of magnetothermal properties and experiments were performed by Watanabe et al. (2004). 

Also the formalism of a model of a high-temperature superconductor based on the exciton mechanism is presented. 

In the next section i/e will outline a Green s function formalism to obtain the exciton energies. The calculations of S. Suhai on two typical polydiacetylene crystals (PTS with an acetylene-like structure and TCDU with a butatriene-like structure) are presented in section III. In section IV the formalism of a model of a high-temperature superconductor based on the exciton mechanism is presented, and in the final section a conclusion is given. 

Considerable experimental support for the CT exciton mechanism for carrier generation has been provided by work on electric field modulated absorption spectroscopy. As described Sebastian et al (21), the electric field induced change in absorption coefficient k for excitation of a Frenkel exciton may be given to first order by the expression... 

---