Quenching, electronic, theory

A type of molecular resonance scattering can also occur from the formation of short-lived negative ions due to electron capture by molecules on surfrices. While this is frequently observed for molecules in the gas phase, it is not so important for chemisorbed molecules on metal surfaces because of extremely rapid quenching (electron transfer to the substrate) of the negative ion. Observations have been made for this scattering mechanism in several chemisorbed systems and in phys-isorbed layers, with the effects usually observed as smaU deviations of the cross section for inelastic scattering from that predicted from dipole scattering theory. 

A semi-classical treatment171-175 of the model depicted in Fig. 15, based on the Morse curve theory of thermal dissociative electron transfer described earlier, allows the prediction of the quantum yield as a function of the electronic matrix coupling element, H.54 The various states to be considered in the region where the zero-order potential energy curves cross each other are shown in the insert of Fig. 15. The treatment of the whole kinetics leads to the expression of the complete quenching fragmentation quantum yield, oc, given in equation (61)... 

The application34 of time-dependent diffusion theory to the quenching constant kCMTf for the perylene-dimethylaniline system in acetonitrile, which exhibits a pronounced dependence on quencher concentration, provides a value of 6 A for the quenching encounter diameter as the adjustable parameter which considerably exceeds that of 3 A computed for the same system in benzene. This accords with the conclusions of Leonhardt and Weller35 that electron transfer may occur with direct contact of donor and acceptor in polar media (Section V.E). 

The remainder of this section considers several experimental studies of reactions to which the Smoluchowski theory of diffusion-controlled chemical reaction rates may be applied. These are fluorescence quenching of aromatic molecules by the heavy atom effect or electron transfer, reactions of the solvated electron with oxidants (where no longe-range transfer is implicated), the recombination of photolytically generated radicals and the reaction of carbon monoxide with microperoxidase. 

It has been observed that the quenching cross section for the diatomic homonuclear molecules N2, H2, and D2, clearly depends on the laser polarization, although to a lesser extent than the electron-scattering intensities.116 Although, in principle, the same discussion may be applied as in electron scattering and the theory of the measurement102 may be applied adequately, heavy-particle collisions, especially with molecules, bring a number of complications that have to be taken into consideration ... 

Electron transfer may also dominate the excited state chemistry of open shell radical ions. The fluorescence of the radical anions of anthraquinone and 9,10-dicyanoanthracene and the radical cation of thianthrene are quenched by electron acceptors and donors, respectively, although detailed kinetic analysis of the electron exchange do not correspond exactly either with Weller or Marcus theory (258). The use of excited radical cations as effective electron acceptors represents a... 

Excited state potentials can also be estimated from kinetic studies of electron transfer quenching reactions involving a series of acceptors and/or donors with varying potentials. By applying electron transfer theory to the quenching step, in conjunction with the predicted dependence of the quenching rate constant on AG° for the electron transfer reaction, estimates for the redox potentials may be obtained (2 ). These approaches have been used successfully in the evaluation of the redox properties of several metal complexes,... 

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