Strong electron-phonon coupling

The model of the chain of hydrogen atoms with a completely delocalized (metallic) type of bonding is outlined in the preceding section. Intuitively, a chemist will find this model rather unreal, as he or she expects the atoms to combine in pairs to give H2 molecules. In other words, the chain of equidistant H atoms is expected to be unstable, so it undergoes a distortion in such a way that the atoms approach each other in pairs. This process is called Peierls distortion (or strong electron-phonon coupling) in solid-state physics ... 

The above dynamical description of the polymerisation strongly parallels that of nonradiative transitions and this is not accidental althouth the monomer crystal from which the polymeric one is issued, do fluoresce, the polymeric one does not, despite its strong absorption at 2 eV. This strongly indicates efficient nonradiative relaxation of the excitation and strong electron-phonon coupling. 

The symmetry of the electronic surface bands has to allow for strong electron-phonon coupling to distortions of appropriate symmetry. 

A AH < kT has important consequences. As the temperature is lowered to where AHg, kT, strong electron-phonon interactions must manifest themselves. Direct evidence for mode softening and strong electron-phonon coupling in the internal Ty < T < 250 K has been provided by measurements of the Mdssbauer recoiless fraction and the X-ray Debye-Waller factor as well as of muon-spin rotation Therefore, it would be... 

The Seebeck coefficient a becomes temperature-independent only above a temperature Tt, where T, — 300 K for x < 0.1 In magnetite, T, can be identified with the onset of strong electron-phonon coupling. The temperature-independent a shows a continuous evolution from the value for P = 1 described by Eq. (16) at x = 0.1 to that by Eq. (15) for x > 0.8. Although small-polaron formation is observed for x > 0.2, regions apparently persist where multielectron jumps can occur. 

When the electron-phonon interaction is included, there may be a phonon-induced delocalization which further broadens the mobility edge and allows conductivity to occur below Ef,. However, sufficiently strong electron-phonon coupling results in polaron formation in the band tails which suppresses the conductivity in the localized states and restores the discontinuous change in a. ... 

Defect recombination is therefore primarily non-radiative. The standard theories of non-radiative transitions are based on the multiphonon processes described in Section 8.1.2. A temperature-independent capture cross-section of about 10 cm" is characteristic of a defect state with a strong electron-phonon coupling (see Fig. 8.6). 

It has been remarked (2 7) that the smallest mobilities tend to show an activated behavior, intermediate ones the weak temperature dependences just discussed, and larger ones a more rapid decrease like T with n=2.5-3. As noted in section IV, activated behavior requires very strong electron-phonon coupling, but it is not obvious v iy this might be present in the very low mobility crystals studied, electrons in orthorhombic sulphur (28) and g-nitrogen (29) and holes in y-oxygen (.29). At the opposite... 

Polymeric species Pt(X"Bn3)2 =C (C=C)m-C C and Pt(X Bn3)2-(C=C) 6H4-(C=C) -ju (m = 0, 1) have been synthesized. The absorption and emission spectra of these complexes showed extended tt-electron conjngation throngh the metal sites on the chain, with a lower itt-itt energy gap for the triacetylenic than for the diacetylenic polymers. Well-resolved vibronic stracture associated with the v(C=C) frequency was observed in both the absorption and emission spectra, indicating strong electron-phonon coupling for these polymers. 

The INS data confirm the electron-phonon coupling to at least some of the modes. Indeed the most remarkable difference between the KsCm, and C o spectra is the disappearance of the and H "" modes at 54 and 196 meV, consistent with strong electron-phonon coupling. This is an important difference from alkali-intercalated graphite, where the corresponding buckling H, " mode does not couple to the ir-electrons because of symmetry considerations. Radial H, and... 

For both PF2/6 and PF8 the aforementioned main chain characteristics are essentially identical and so any pronounced differences are likely to originate in secondary structural characteristics of the functionalizing side chains. PF8 studies by Bradley and coworkers [16] first identified the unusual spectroscopic emission band now conventionally referred to as the phase . The hallmark signature of this peculiar chain structure is a relatively sharp series of emission bands red shifted some lOOmeV from those seen when the polymer is prepared in a glassy state, tt-Conjugated polymers have strong electron-phonon coupling and so, in addition to the it-it emission, there is a manifold of vibronic overtones spaced approximately 180 meV apart and red-shifted from the dominant n-n emission band. 

Next, we discuss the concept of phonon-assisted reactions. In relation to thermal reactions, they can be assisted by phonon-mode softening leading to large-amplitude overdamped oscillations. In the case of a photochemical reaction, a strong electron-phonon coupling can assist in polymerization. Then some non-linear spectroscopic studies are discussed which illuminate on the dynamics of photopolymerization process. Then follows a discussion of results on reaction in a different kind of molecular assembly, the Langmuir-Blodgett films. Finally, some gas-solid interface reactions which produce polymers in a doped state are discussed. 

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. 

The information on the formation of a polaron or an excimer is derived from the low temperature electronic absorption and emission spectra of the reactive crystals. The strong electron-phonon coupling in the reactive state manifests itself as a very strong phonon-side band In the liquid helium temperature spectra. 

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