Electron tunneling, evidence

An essential step forward was also the development of kinetic models for electron tunneling reactions in solids [20-25]. Kinetic equations corresponding to these models were found to describe experimental data rather accurately. The agreement of experimental data with theory together with the absence of the temperature dependence for the reaction rate (which rules out its control by thermal diffusion) and with the evidence of considerable... 

Recombination reactions of trapped electrons with hole centres were the first chemical processes for which chemists succeeded in getting reliable experimental evidence for their occurring via electron tunneling over a large distance. Unfortunately, however, the initial distribution over distances between the reacting particles in the electron-hole centre pairs is, as a rule, known only approximately. This circumstance hinders considerably the detailed quantitative comparison of the kinetics observed with that theoretically expected for tunneling reactions. 

Thus, the results obtained by Huddleston and Miller [91] confirm the possibility of the quantitative prediction of etr decay kinetics by reactions with bifunctional acceptors on the basis of the data on the et decay kinetics by reactions with isostructural monofunctional acceptors. These results present further convincing evidence in favour of the validity of the model of direct long-range electron tunneling for low-temperature reactions of etr. 

As already noted above, the first experimental evidence of electron tunneling from excited donor particles to acceptors was obtained independently in Refs. [37] and [38]. 

Evidence for the dual proton and electron tunneling process required for simultaneous electron and proton transfer has been obtained from both enzymatic and model systems. In biomolecular systems, the strongest evidence for proton tunneling processes is obtained from kinetic isotope effects, where both anomolously high protium-tritium kinetic isotope effects and distinct temperature profiles for these isotope effects can be used as diagnostic tools for proton tunneling events [57]. 

V +] or when sensitizers that did not bind to the PC membrane at all were present in the external medium. Additional studies using C-radiolabeled (C7)2V + demonstrated that transmembrane diffusion of the viologen did occur on relevant time-scales, and a full kinetic analysis of apparent transmembrane reduction of vi-ologens across PC liposomal membranes provided direct evidence for the proposed viologen-mediated transmembrane redox pathway [109], Thus, in retrospect, it is clear that these reactions did not occur by electron tunneling across the bilayer. 

The evidence for the atom is now direct, as it is possible to see atoms directly, using such techniques as electron tunnelling microscopy. If this technique is used to look at the surface of copper metal, the atoms show up as bumps (Figure 2.1). The atom may be defined as the smallest unit of an element that retains the physical and chemical characteristics of the element. Dalton considered that the atom could be treated as a hard sphere that could not be broken down into smaller units, i.e. it had no internal structure, rather like a billiard ball. While this is not quite true, it can be understood in terms of the present knowledge of the structure of the atom. In the late 1800s, J. J. Thompson showed that the atom was built up from much smaller units, namely, electrons, protons and neutrons (Table 2.1). 

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