Electron photoemission

Electron work functions of metals in solution can be determined by measurements of the current of electron photoemission into the solution. In an electrochemical system involving a given electrode, the photoemission current ( depends not only on the light s frequency v (or quantum energy hv) but also on the potential E. According to the quantum-mechanical theory of photoemission, this dependence is given by... 

Electron photoemission from an electrode into an electrolyte solution, which yields solvated (hydrated) electrons, can be regarded as a particular case of reactions with photoexcitation of the electrode (Section 29.2). 

For phenomena involving electrons crossing the phase boundary (photocurrents, electron photoemission), the quantum yield j of the reaction is a criterion frequently employed. It is defined as the ratio between the number of electrons, N, that have crossed and the number of photons, that had reached the reaction zone (or, in another definition, the number of photons actually absorbed by the substrate) J=N /N. ... 

Events of electron photoemission from a metal into an aqueous solution had first been documented in 1966 by Geoffrey C. Barker and Arthur W. Gardner on the basis of indirect experimental evidence. The formation of solvated electrons in nonaque-ous solutions (e.g., following the dissolution of metallic sodium in liquid ammonia) had long been known, but it was only in the beginning of the 1950s that their existence in aqueous solutions was first thought possible. It is probably for this reason that even nowadays in aqueous solutions we more often find the term solvated than hydrated electrons. 

The basic law of electron photoemission in solntions which links the photoemission current with the light s frequency and with electrode potential is described by Eq. (9.6) (the law of five halves). This eqnation mnst be defined somewhat more closely. As in the case of electrochemical reactions (see Section 14.2), not the fnll electrode potential E as shown in Eq. (9.6) is affecting the metal s electron work function in the solution bnt only a part E - / ) of this potential which is associated with the potential difference between the electrode and a point in the solntion jnst outside the electrode. Hence the basic law of electron photoemission into solntions should more correctly be written as... 

Semiconductor electrodes exhibit electron photoemission into the solution, like metal electrodes, but in addition they exhibit further photoelectrochemical effects due to excitation of the electrode under illumination. The first observations in this area were made toward the middle of the twentieth century. At the end of the 1940s,... 

When semicondnctors are irradiated with photons of high energy, electron photoemission is possible, as in the case of metals. When the photon energy is lower than the electron work function in the solution, under given conditions, but is still higher than the semiconductor s bandgap W, ... 

Brodsky, A. M., and Yu. V. Pleskov, Electron photoemission at a metal-electrolyte solution interface, Progr. Surf. Sci., 2, 1 (1972). 

Figure 6. The energy level structure for an n-semiconductor-electrolyte interface as is appropriate for electron photoemission.

Now the binding energy corresponding to the center of gravity of the ith electron photoemission spectrum is... 

All photoeffects involve the absorption of photons to produce an excited state in the absorber or liberate electrons directly. With the direct release of electrons, photoemission may occur from the surface of solids. While the excited state may revert to the ground state, it may proceed further to a photochemical reaction to provide an electron-hole pair (exciton) as the primary photoproduct. The exciton may dissociate into at least one free carrier, the other generally remaining localized. In an externally applied electric field, photoconduction occurs. Photomagnetic effects arise in a magnetic field. Absorption of photons yield photoelectric action spectra which resemble optical absorption spectra. Photoeffects are involved in many biological systems in which charge transfer takes place (e.g., as observed in the chlorophylls and carotenoids) [14]. 

As shown in Table 25-2, there are two general types of transducers one type responds to photons, the other to heat. All photon detectors are based on the interaction of radiation with a reactive surface either to produce electrons (photoemission) or to promote electrons to energy states in which they can conduct electricity (photoconduction). Only UV, visible, and near-IR radiation possess enough energy to cause photoemission to occur thus, photoemissive detectors are limited to wavelengths shorter than about 2 p.m (2000 nm). Photoconductors can be used in the near-, mid-, and far-IR regions of the spectrum. 

Figure 2.8 (a) Schematic diagram of electron photoemission from a semiconductor. One pulse (hVi)... 

Solvated electrons were considered to be exotic over several decades and that is why comparatively little time was devoted to their study. However, the situation drastically changed early in the 60 s when the crucial role of solvated electrons in radiation chemistry was reliably ascertained. This triggered numerous theoretical and experimental studies into the structure and properties of solvated electrons and their solutions, the pathways of formation of solvated electrons, the kinetics and the mechanism of reactions involving their participation. The findings of these studies have been summarized in several monographs and in the proceedings of international conferences dedicated to the physico-chemistry of solvated electrons (the last, 6th Weyl Symposium was held in 1983) therefore, these will not be spjecific-ally considered in this overview. We shall juxtapose these with electrochemical data, of course. The same relates also to electron photoemission from metal into solution, to which monographs are devoted. 

Electron photoemission from solvated electron solution (in solvents such as hexa-methylphosphotriamide and liquid ammonia the solvated electrons are fairly stable) to vapour phase has been studied by Delahay and co-workers (whose works are reviewed in Ref. ). According to them, this process proceeds in three stages solvated electron photoionization diffusion of generated delocalized electrons to the solution s surface and emission proper, i.e. transition of electrons to the vapour phase where they are transferred from the cathode surface (i.e., from the solution) to the anode by the external electric field. 

If the electrode process is not complicated by adsorption or other phenomena, depending directly on the nature of the electrode, then the rate of the charge transfer stage proper should be independent of the electrode material. Indeed, this type of independence has been earlier ascertained experimentally for the electroreduction of anions and the electron photoemission into solution, fo such processes should belong also the cathodic generation of solvated electrons by the primary mechanism. 

B. Vogt, B. Schmiedeskamp, and U. Heinzmann. Spin-Resolved Core and Valence Electron Photoemission from Non-epitaxially Grown Pb Layers on Pt(lll). Vacuum 41 1118 (1990). 

Inner-shell photoionization of atoms, molecules, clusters, and the condensed phase cannot be simply described by one electron photoemission, assuming frozen orbital energies. This simple approach, which corresponds to Koopmans theorem, is often successfully applied to describe valence-shell photoionization. However, this approach completely fails for inner-sheU photoionization, where deviations of the order of 10-20 eV relative to the experimental results are found. 

PHOTOEXCITATION OF METALS (ELECTRON PHOTOEMISSION INTO SOLUTIONS)... 

The solvation free energy of an electron in polar solvents can be estimated by electron photoemission (EPE) [Ya 77]. Imai and Yamashita [Im 78] suggested therefore the use of the solvation free energy of the electron for the characterization of the electrophilic properties (acidity) of solvents. 

Margulis has shown how general arguments based on the K-electron photoemission spectrum near the conduction and valence band energies can lead to an estimation of y for carbon nanotubes, provided it is assumed that virtual jT-electron transitions between band states are responsible for the effect. 

The linear relationship between the work function and the potential was corroborated both for electron photoemission and for dark cathodic generation of electrons in solutions in aprotic solvents in which sufficiently high negative potentials can be achieved, i.e., sufficiently low values of the work function can be obtained even without illumination. These low values allow a substantial rate of electron emission to arise. These experiments have also demonstrated that the effective work function is independent of the nature of the metal (Figure 4). 

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