Double layer photoemission

Photoemission phenomena are of great value for a number of areas in electrochemistry. In particnlar, they can be used to study the kinetics and mechanism of electrochemical processes involving free radicals as intermediates. Photoemission measurements can be also used to study electric double-layer structure at electrode surfaces. For instance by measuring the photoemission current in dilute solution and under identical conditions in concentrated solutions (where we know that / = 0), we can find the value of / in the dilute solution by simple calculations using Eq. (29.9). 

Vq values are of theoretical and practical importance. In the language of physics, Vq is identified as the bottom of the conduction band, while in chemical terms, it is called the electron affinity of the liquid. As discussed in Section 7.6, Vq reflects the subtle balance of repulsive and attractive forces acting on the electron. The more negative the value, the greater the influence of attraction, while positive Vq values point toward the decisive influence of repulsion. Vq data are compiled in Table 3. The measured data were obtained either by the photoelectric method (see Section 6.2), by electron emission from liquids, or by comparison of photoconductivity and photoemission data (see Section 6.5). Data measured by the photoelectric effect may suffer from the effect of a charged double layer at the metal liquid interface. In principle, the values obtained by the electron emission method or from the comparison of photoconductivity and photoemission thresholds should be free from this ambiguity. 

There are two important differences between photoemission into vacuum and into condensed media. Firstly, the work function in the latter case usually (but not necessarily) is lowered because of an interaction of the photoelectron with the solvent. Further, chemical reactions are possible with deliberately introduced scavengers or trace impurities present in an electrolyte. Secondly, the presence of an electrical double layer introduces a potential drop across the Helmholtz plane and diffuse layer. In some earlier theories, the electrical double layer in the presence of solute molecules was thought to screen effectively the positive charge remaining after the electron is emitted (unlike the case of emission into vacuum), thereby reducing image effects and changing the theoretical form of the photocurrent rate expressions. 

Two questions can be asked and will be examined in this section firstly, how does the electrical double layer at an electrode affect the photoemission process, and secondly, as one of the key aims of electrochemical experimentation is to clarify our understanding of the structure and function of the double layer, can photoemission studies assist with determining its role in heterogeneous equilibrium and dynamic processes Experiments have been performed from each of these viewpoints and these topics have been represented in detail elsewhere. ... 

As relatively less is known concerning electrical double layers at solid and semiconductor electrodes than at liquid mercury electrodes, it is not surprising that the majority of photoemission double-layer studies have been made at the latter. A detailed and clear account of the double layer itself has been given by Mohilner. The photoemission influences broadly may be classified as primary if the electron emission step is affected directly, or secondary if subsequent reactions of the solvated electron with homogeneous acceptors in solution and/or the electrode are modified. 

When the double-layer thickness does not exceed the de Broglie wavelength of the emitted electron and in the absence of specifically adsorbed ions and organic adsorbates, the interfacial structure has little influence on photoemission directly. In other words, the double layer in concentrated electrolytes is transparent and the electrons can tunnel through the inner and outer (diffuse) layers. Experimentally, variation of the electrolyte concentration using an uncharged scavenger (N2O) permits two effects to be resolved. At... 

When single-stranded DNA molecules, which do not form a well-organized layer, were adsorbed on a gold substrate, no asymmetry in the photoemission yield for clockwise or counterclockwise circular polarized light could be observed. However, when double-stranded DNA, which formed a well-ordered monolayer, was adsorbed, a large asymmetry was observed. 

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