Dimeric lattice distortion

The argument, just briefly outlined, show that, in a quarter-filled system, the presence of strong on-site correlations leads to deviations of the optical properties fi om the Drude behavior [3] expected for independent electrons. Such deviations are not as large, as those predicted for a half-filled system, and, in particular, do not relate directly to U itself, but, rather, to dimeric lattice distortions and, possibly, to the nearest-neighbor repulsion V. 

In a system that has undergone a Peierls distortion, the partially dimerized lattice of atoms will have a different density of electrons in the region between the close partners than between more distant neighbors. As a result, an additional modulation will occur in the density of the electrons of the distorted chain compared to that of the undistorted chain. 

Quenching Excimers and Exciplexes.—By measurements of decay times and fluorescence anisotropy of pyrene and the excimer in cellulose acetate films it has been found that the medium consists of spaces where small pyrene molecules have considerable freedom, Dissado and Walmsley have developed a complete theory of excimer formation and exciton-induced lattice distortion in crystals. Reference is made to data on 9-cyanoanthracene. The spectroscopy of chemically linked dimers of l,3-(l,l -dinaphthyl)propane in a... 

In Rb+TCNQ and TMB /tri-methyl-benzim-idazolium/TCNQ, the former having a dimerized chain-type TCNQ"-structure, the latter having isolated dimers, such triplet excitons were studied. The observed spin-dipolar interaction gives information on the electron-hole separation, the behavior of the linewidths as a function of temperature and angle of the magnetic field yields accurate values of the anisotropy of the exciton motion and shows that the excitons are self-trapped by a lattice distortion. 

The cis-syn thymine dimer from DNA is believed to be formed from intra-strand dimerization of adjacent thymine residues. Photoaddition of two unsaturated molecules in the solid state can arise only if they are initially located in proximity in the crystal lattice [545, 546]. The formation of interstrand dimers would require gross distortion of the helical structure of DNA in order for the bases to approach the limiting distance (c. 4 A.) Hence, such dimers would be formed in only very small amount. However, the composition of the photoproducts may differ under varying experimental conditions [547-550]. 

The physical properties of the thallium halides have been tabulated.1 Crystalline T1F has a distorted rock-salt lattice, with two independent thallium sites.333-335 The thermodynamic properties of the solid have been reassessed.336 At high temperature, the vapor phase contains TLF and T12F2 molecules,337 and matrix isolation studies have demonstrated the presence of both T1F and T12F2 in the solid the dimers have linear symmetry.338... 

Therefore, it is modulated by the intermolecular displacement occurring in a phonon motion. Photochemical aggregation reactions, such as dimerization or polymerization reactions, can be assisted by the occurrence of strong electron-phonon interaction in the reactive electronic state. This strong electron-phonon interaction creates a local lattice-deformation in the reactive (excited) electronic state. The deformation traps the electronic excitation and, at the same time, it may provide a local preformation of the product lattice if the distortion is along the reaction co-ordinate. Both these features assist a photochemical aggregation reaction. 

Depending on the strength of the electron-phonon coupling, one may observe the formation of a polaron or an exclmer. The formation of a polaron does not lead to the loss of the identity of the monomer in the excited state, but simply the excitation is localized by local lattice-deformation. The excimer formation requires a severe distortion oi the local structure which leads to an excited state dimer. It may also be pointed out that the polaron mechanism is a purely dynamic effect which can occur even in a defect-free lattice. In contrast, the excimer formation can occur either by a dynamic effect due to strong electron-phonon coupling or by a static effect due to sites deformed by the presence of defects. 

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