Coupling mechanism electronic excitations

Formally, one can think of the Raman transition probability being proportional to the elements of the polarizability tensor of a bound electron as the exciting frequency approaches the resonance frequency, these elements are enhanced in a Lorentz model of the bound electron. A common example of this mechanism is furnished by the ring-breathing (in-plane expansion) modes of porphyrins. Another mechanism, called vibronic enhancement, involves vibrations which couple two electronic excited states. In both mechanisms, the enhancement factors are nearly proportional to the intensities in the absorption spectrum of the adsorbate. 

The electrical conduction behaviour in doped polymers has been extensively studied and new conduction mechanisms involving coupling of electronic excitations to non-linear conformations have been proposed. The conduction behaviour has been explained by dopant-induced bond-alternation defects such as soli-tons, polarons and bipolarons [1,2,9,11,32-35]. Thermal treatment of organic monomers and polymers results in the formation of extended aromatic structures or planar ladder structures, and hence the heated polymers have considerably smaller bandgaps and higher mobility than unheated ones. They exhibit large... 

Semiconducting electrodes offer the intriguing possibility to enhance the rate of an electron-transfer reaction by photoexcitation. There are actually two different effects Either charge carriers in the electrode or the redox couple can be excited. We give examples for both mechanisms. 

The binding energies of even low mass adsorbates are sufficiently large so that direct momentum transfer does not cause desorption. This fact, coupled with the observation that ESD ions have large energies indicates that ESD proceeds via an electronic excitation mechanism. The energy dependence of the cross sections are quite consistent with this conclusion . 

The coupling mechanism given above for mixing excited u terms into g electronic terms through vibrational modulation of the ligand field is likely to be less efficient if terms of different spin are involved. In accordance with this, it is observed that spin-forbidden bands are a good deal narrower than are the spin-allowed ones dealt with above, corresponding to reduced overlap of available vibrational structure. Half-widths of a few hundred to one thousand cm-1 seem to be involved. 

Apart from the heat bath mode, the harmonic potential surface model has been used for the molecular vibrations. It is possible to include the generalized harmonic potential surfaces, i.e., displaced-distorted-rotated surfaces. In this case, the mode coupling can be treated within this model. Beyond the generalized harmonic potential surface model, there is no systematic approach in constructing the generalized (multi-mode coupled) master equation that can be numerically solved. The first step to attack this problem would start with anharmonicity corrections to the harmonic potential surface model. Since anharmonicity has been recognized as an important mechanism in the vibrational dynamics in the electronically excited states, urgent realization of this work is needed. 

The second major section will focus on those special centers of minerals thought to be of importance to their catalytic activity, with an emphasis on the known and possible effects of electronic excitation on the population and mode of action of these centers. Metastable states constitute a hidden variable in defective solids, a non-negligible one for non-stoichiometric ones. With regard to concepts of mineral catalysis, the only systems for which extensive spectroscopic information on mineral catalytic centers has been definitively coupled to the mechanism of a well understood surface chemical reaction is exchange on binary oxides. Existing data for the... 

Connection with vibrational lifetime on surfaces. The decay of molecular vibrations in the excitation of the electron-hole pairs of metallic surfaces have been identified with the mechanisms of vibration excitation by tunneling electrons [42]. Intuitively this may seem so. Indeed, an excited vibration may couple to the surface electronic excitations through the same electron-vibration matrix elements of Eqs. (2) and (4). The surface... 

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