Vacuum, electron transfer

In the following we would like to speculate on the maximal rate constant and maximal electron-exchange matrix element possible in pure through-vacuum electron transfer of organic molecules in the face-to-face geometry at van-der-Waals contact, and on a potential adiediatic-to-nonadied)atic transition. The Landau-Zenner parameter used to characterize the... 

References to a number of other kinetic studies of the decomposition of Ni(HC02)2 have been given [375]. Erofe evet al. [1026] observed that doping altered the rate of reaction of this solid and, from conductivity data, concluded that the initial step involves electron transfer (HCOO- - HCOO +e-). Fox et al. [118], using particles of homogeneous size, showed that both the reaction rate and the shape of a time curves were sensitive to the mean particle diameter. However, since the reported measurements refer to reactions at different temperatures, it is at least possible that some part of the effects described could be temperature effects. Decomposition of nickel formate in oxygen [60] yielded NiO and C02 only the shapes of the a—time curves were comparable in some respects with those for reaction in vacuum and E = 160 15 kJ mole-1. Criado et al. [1031] used the Prout—Tompkins equation [eqn. (9)] in a non-isothermal kinetic analysis of nickel formate decomposition and obtained E = 100 4 kJ mole-1. 

The common example of real potential is the electronic work ftmction of the condensed phase, which is a negative value of af. This term, which is usually used for electrons in metals and semiconductors, is defined as the work of electron transfer from the condensed phase x to a point in a vacuum in close proximity to the surface of the phase, hut heyond the action range of purely surface forces, including image interactions. This point just outside of the phase is about 1 pm in a vacuum. In other dielectric media, it is nearer to the phase by e times, where e is the dielectric constant. 

In the present chapter, all values of Jj, jl, X, Wp, and Up refer to a single electron (or single species) and are stated in electron volts, as in Section 9.1. We recall that the values of work functions are always positive hence, the values of and are always negative (electron transfer from vacuum into a metal is associated not with an expenditure but with a gain of energy). 

Constant A in Eqs. (29.5) and (29.6) is about 4.4 eV when the standard hydrogen electrode is used as the reference electrode. This value has been determined from experimental values for the electron work function of mercury in vacuum, which is 4.48 eV, and for the Volta potential, between the solution and a mercury electrode polarized to = 0 V (SHE), which is -0.07 V (the work of electron transfer is 0.07 eV). The sum of these two values, according to Eq. (9.8), corresponds to the solution s electron work function at this potential (i.e., to the value of constant A with an inverted sign). 

At electrode potentials more negative than approximately - 2.8 V (SHE), free solvated electrons appear in the solution as a result of (dark) emission from the metal. At this potential the electrochemical potential of the electrons according to Eq. (29.6) is about —1.6 eV, which is at once the energy of electron hydration in electron transfer from vacuum into an aqueous phase. 

Photo-oxidation of carotenoids in Ni-MCM-41 produces an intense EPR signal (Figure 9.11) with -value 2.0027 due to the carotenoid radical another, less intense EPR signal, with =2.09 is attributed to an isolated Ni(I) species produced as a result of electron transfer from the carotenoid molecule to Ni(II). It has been reported that Ni(I) ions prepared upon reduction of Ni(II)-MCM-41 by heating in a vacuum or in dry hydrogen exhibits an EPR spectrum with , =2.09 and N=2.5... 

The methodology of surface electrochemistry is at present sufficiently broad to perform molecular-level research as required by the standards of modern surface science (1). While ultra-high vacuum electron, atom, and ion spectroscopies connect electrochemistry and the state-of-the-art gas-phase surface science most directly (1-11), their application is appropriate for systems which can be transferred from solution to the vacuum environment without desorption or rearrangement. That this usually occurs has been verified by several groups (see ref. 11 for the recent discussion of this issue). However, for the characterization of weakly interacting interfacial species, the vacuum methods may not be able to provide information directly relevant to the surface composition of electrodes in contact with the electrolyte phase. In such a case, in situ methods are preferred. Such techniques are also unique for the nonelectro-chemical characterization of interfacial kinetics and for the measurements of surface concentrations of reagents involved in... 

Fig. 4-11. Energy diagram for electron transfer from a standard gaseous electron across a solution/vacuum interface, through an electrolyte solution, and across a metal/solution interface into a metal electrode = real potential of electrons e,s) in electrolyte...

Further, the electron level of adsorbed particles differs from that of isolated adsorbate i>articles in vacuum as shown in Fig. 5-5, this electron level of the adsorbate particle shifts in the course of adsorption by a magnitude equivalent to the adsorption energy of the particles [Gomer-Swanson, 1963]. In the illustration of Fig. 5-5, the electron level of adsorbate particles is reduced in accordance with the potential energy curve of adsorption towards its lowest level at the plane of adsorption where the level width is broadened. In the case in which the allowed electron energy level of adsorbed particles, such as elumo and ehcmio, approaches the Fermi level, ep, of the adsorbent metal, an electron transfer occurs between... 

It has been argued [235] by analogy with the case of molecules adsorbed on glassy n-hexane [232] that this enhancement is due to the electron transfer to CF2CI2 of an electron previously captured in a precursor state of the solvated electron in the water layer, which lies at and just below the vacuum level [300,301] and the subsequent. Similar results have been reported for HCl adsorbed on water ice [236]. It has been proposed that enhanced DEA to CF2CI2 via electron transfer from precursor-solvated states in ice [235] may explain an apparent correlation between cosmic ray activity (which would generate secondary LEE in ice crystals) and atmospheric ozone loss [11]. The same electron transfer mechanism may contribute to the marked enhancement in electron, and x-ray-induced dissociation for halo-uracil molecules is deposited inside water ice matrices [39]. 

We discuss this with help of Fig. 2 for a cyclic electron transfer from a molecule in the ground state to the vacuum level, and back to the ionized molecule. After each electron transfer we leave enough time for relaxation of the solvent to reach the new equilibrium state of interaction in which electrostatic forces play a prom-... 

Electron transfer is a fast reaction ( 10-12s) and obeys the Franck-Condon Principle of energy conservation. To describe the transfer of electron between an electrolyte in solution and a semiconductor electrode, the energy levels of both the systems at electrode-electrolyte interface must be described in terms of a common energy scale. The absolute scale of redox potential is defined with reference to free electron in vacuum where E=0. The energy levels of an electron donor and an electron acceptor are directly related to the gas phase electronic work function of the donor and to the electron affinity of the acceptor respectively. In solution, the energetics of donor-acceptor property can be described as in Figure 9.6. 

The distance dependence of electron transfer has been studied extensively for the homogeneous case. An approximately exponential decay of the electronic coupling has been found with the number of saturated bonds in the spacer unit (see for example [6,7]). The results presented here suggest that an exponential dependence fits also our data for heterogeneous electron transfer in ultra-high vacuum. A different result has been reported for electron transfer from Re complexes to anatase where a local triplet state can play a role [8]. 

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