Electron induced processes

Electrocatalysis is an important application of electron-induced processes. In electrocatalysis the catalyst has at first to accept chaige(s) from the electrode, and thereafter catalysis can take place. Enzyme-immobilized electrodes are tj ical examples used for various biosensors as well as for investigation of fundamental biocatalysis. The enzymatic active center is often located inside a protein molecule, so that mediation of charges from the electrode surface to the active center is important. Viologens, metallocenes and other metal complexes have been used as such mediators. 

Self-contained in a high-temperature high-density plasma is the electron thermal energy per unit voliune NekTe (Ne aud are the electron density and temperature, respectively). This intrinsic energy has been tapped veiy successfully for pumping X-ray laser transitions through inelastic collisions of free electrons with ions. Excitation, recombination, and (innershell) ionization are all possible electron-induced processes for effective pumping. The first two have proven to be most successful to date. 

Fig. 14. Schematic of the Auger electron emission process induced by creation of a K level electron hole.

The X-ray emission process followii the excitation is the same in all three cases, as it is also for the electron-induced X-ray emission methods (EDS and EMPA) described in Chapter 3. The electron core hole produced by the excitation is filled by an electron falling from a shallower level, the excess energy produced being released as an emitted X ray with a wavelength characteristic of the atomic energy levels involved. Thus elemental identification is provided and quantification can be obtained from intensities. The practical differences between the techniques come from the consequences of using the different excitation sources. 

Another class of photochemically relevant polyphosphazenes is formed by macromolecules having chromophores able to absorb light in a selective way and to transfer it to external species, thus inducing different reactions by energy transfer processes. In some cases electron transfer processes are also involved. These situations are described by Formula below and the corresponding polymers and external reagents are reported in Table 26. 

Lantz, J. M. and Corn, R. M. (1994) Electrostatic field measurements and hand fiattening during electron-transfer processes at single-crystal Ti02 electrodes by electric field-induced optical second harmonic generation. J. Phys. Chem., 98, 4899-4905. 

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

J. Tauc, Time-Resolved Spectroscopy of Electronic Relaxation Processes P.E. Vanier, IR-Induced Quenching and Enhancement of Photoconductivity and Photoluminescence... 

In this light we will now consider some examples of electrochemically detectable molecular reorganisations induced by electron transfer processes, which can be chemically more appealing than the simple variations of bond distances/angles discussed up to now. 

Electron transfer processes induce variations in the occupancy and/or the nature of orbitals which are essentially localized at the redox centers. However, these centers are embedded in a complex dielectric medium whose geometry and polarization depend on the redox state of the system. In addition, a finite delocalization of the centers orbitals through the medium is essential to-promote long-range electron transfers. The electron transfer process must therefore be viewed as a transition between two states of the whole system. The expression of the probability per unit time of this transition may be calculated by the general formahsm of Quantum Mechanics. 

Many-electron models give a better description of the change of electronic state induced by the electron transfer process, because they are able to account for effects involving a large set of valence electrons. However, the functions vl/3 and vj/b are then necessarily antisymmetric with respect to these electrons, so that the calculations become much more complicated than in one-electron treatments. 

Some authors have described the time evolution of the system by more general methods than time-dependent perturbation theory. For example, War-shel and co-workers have attempted to calculate the evolution of the function /(r, Q, t) defined by Eq. (3) by a semi-classical method [44, 96] the probability for the system to occupy state v]/, is obtained by considering the fluctuations of the energy gap between and 11, which are induced by the trajectories of all the atoms of the system. These trajectories are generated through molecular dynamics models based on classical equations of motion. This method was in particular applied to simulate the kinetics of the primary electron transfer process in the bacterial reaction center [97]. Mikkelsen and Ratner have recently proposed a very different approach to the electron transfer problem, in which the time evolution of the system is described by a time-dependent statistical density operator [98, 99]. 

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