Electronic-vibrational coupling parameters

Ab initio Determination of Electronic-Vibrational Coupling Parameters... 

Earlier theoretical work on the excited-state dynamics of pyrazine focused on the vibronically induced anharmonic couplings in the Si state, on the interactions between gerade and ungerade rm states or on selected PE functions and electron-vibrational coupling parameters. In a series of papers the Munich group has characterized the S2 and 51-state PE surfaces in the conformational subspace relevant for the short-time photophysics following the S2 — So and 5i — So transitions.In the course of this work, the treatment has steadily improved, as regards the accuracy of the electronic-structure calculation as well as the number and description of the vibrational modes considered. The most accurate calculations are based on the CASSCE/MRCI method with a basis set of DZP quality. 

Intramolecular vibrations strongly scatter electrons near the Femii-surfacc in doped fuUerenes. A simple expression for the electron-phonon coupling parameters for this case is derived and evaluated by quantum-chemical calculations. The observed superconducting transition temperatures and their variation with lattice constants can be understood on this basis. To test the ideas and calculations presented here, we predict that high frequency Hg modes acquire a width of about 20% of their frequency in superconductive foUerenes, and soften by about 5% compared to the insulating fuUerenes. 

The exciting discovery of super-conductivity in metallic fiillerencs (f) leads us to inquire whether the classic mechanism for superconductivity, namely, effective electron-electron attraction via the interaction of electrons with vibrations of the ions, is applicable here as well. Associated with this is the question of whether the direct electron-electron repulsion in FuUerenes can suppress conventional singlet pairing. In this paper we exploit the special nature of cluster compounds to derive a particularly simple expression for electron-vibrational coupling from which parameters of the superconducting state of fuUerenes are easily calculated. Further, we present arguments why the effective repulsions in fuUerenes are no different than in conventional metals. 

Fig. 2. Relative intensities of the electronic-vibrational lines for different values of the coupling parameter S. The envelope of the lines yields the bandshape...

Fig. 6.9. Left-hand side Vibrational excitation function N(ro) and weighting function W(ro) versus the initial oscillator coordinate ro for three values of the coupling parameter e. The equilibrium separation of the free BC molecule is f = 0.403 A and the equilibrium value within the parent molecule is re = 0.481 A. Right-hand side Final vibrational state distributions P(n) for fixed energy E the quantum mechanical and the classical distributions are normalized to the same height at the maxima. The classical distributions are obtained with the help of (6.32). The lowest part of the figure contains also the pure Franck-Condon (FC) distribution (

Figure 8 Vibrational potential energy vs. configurational coordinate for electron transfer via vibronic coupling betweeg two symmetric states of a single oscillator A, initial state B, vibration-ally excited state B, thermally excited state X9 vibronic coupling parameter E f optical transition energy E t thermal transition energy (Reproduced with permission from Ref 33 Copyright 1987 The Clay Minerals Society) ...

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