Intramolecular phonons

The key to understand the anomalous behaviors in A vC,o is the electron-electron interaction, U, and the electron-phonon interaction, S. In A vCgo, the important interaction as U is the Coulomb repulsion between the flu electrons. The importance of U has been pointed out based on the results of the photoemission experiments [15]. Also, in AxC o, the important interaction as S is the coupling of the tlu electrons to the intramolecular phonons of the CRaman experiments [16], Nevertheless, a complete understanding of the anomalous behaviors in AxCgo has not been established yet. The reason is that the system with the orbital degree of freedom in which both U and S are important has not been known so far [17-20],... 

In this section, a model which gives the basis of the present study is introduced to investigate the electronic properties of A,Cfio [17]. First, the one-electron part of the Hamiltonian, which describes the itinerant motion of the flu electrons in terms of the electron transfer T, is given. Next, the electron-electron interaction U and the electron-phonon interaction S are examined U represents the Coulomb repulsion between the t u electrons and S represents the coupling of the fiu electrons to the intramolecular phonons of the Cdynamical aspect of S is pointed out. 

We investigate ID lattice of spinless double degenerated electron states linearly coupled to two intramolecular phonon modes described by Hamiltonian... 

We speculated previously that superconductivity in KjCso and Rb3Ceo resulted from electron pairing mediated by high-frequency intramolecular phonons of the Qo molecule, with the change in between KaCeo and RbaCso explained by a change of the density of states at the Fermi level by about 10%. In this weak-coupling BCS model, the variation of is given by... 

Our data provide strong evidence for the presence of election-phonon coupling to H, modes of KjC o, as predicted by intramolecular phonon theories . Higher-resolution data and detailed mode assignments are needed for a more complete picture, particularly the location of the softened H, modes in... 

It follows from the above discussion that considering excitons and intramolecular phonons as independent particles is approximate. It is clear, in particular, that a sufficiently strong exciton-phonon interaction can create propagating bound states, when electronic and vibronic excitations are centered on the same molecule. These states correspond to the previously discussed weak resonant interaction case. But the existence of such states in the vibronic spectrum does not exclude the existence of free excitons and intramolecular phonons states. Both types of states can usually coexist in the vibronic spectrum, in analogy to the case of two interacting particles, where continuum states, corresponding to free particles, coexist with bound states. 

If the exciton-phonon coupling in organics is dominant we can neglect in eqn (13.113) the contribution of exciton-phonon coupling in semiconductor QWs. Then to populate the states of branch p = 2 with large radiative width (i.e. states with small k < 0.2 kcav, see Fig. 13.14c,d) we assume a dominant, resonant, intramolecular phonon to play the main role so that... 

This case is of physical interest especially in connection with the interaction of electrons with intramolecular phonons. 

Kato, T. and Yamabe, T., Electron-intramolecular phonon coupling and possible superconductivity in negatively charged coronene and corannulene, J. Chem. Phys., 117, 2324, 2002. 

The (TTF" )2 dimer that we have used to test our spectral predictions appears to be well suited as a prototype system. In fact,the electron transfer interaction between TTF " cation radicals is strong,and the high symmetry of the dimer (effectively D2fc) prevents any mixing of the CT and the LE excitations. We emphasize that the selective RRS enhancement of the intramolecular phonon modes is a distinctive feature of the present model and test case. It occurs under the restrictive conditions that (i) the dimer is symmetric (ii) the CT states do not mix with localized molecular excitations. When any of these two conditions is broken,the intramolecular modes can participate in the RRS enhancement. 

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