Photoelectron Production

Photoelectron production [Eq. (98)] is an important photochemical process for several transition metal complexes2,24S)  

In principle, this is the simplest electron transfer reaction of an excited state. In practice, complications arise because of the very peculiar nature of the solvated electron246,247 The way in which the electron separates from its parent molecule and the fate of the separated electron have been the object of much discussion. 

Photoelectron ejection is for some molecules connected with the reaction of a solvated electron with other species, as it is known that in several cases this reaction leads to the formation of electronically excited states [Eq. (99)]. Whether reaction (99) or (100) prevails, when reaction (100) is very exoergonic, is of course important for a discussion of Marcus or anti-Marcus behavior (Section 6). 

Although the formation of hydrated electrons upon UV irradiation of Fe(CN)4-was first reported in 1963248), in 1970 there was still a substantial disagreement on 

Recently, Meisel et al.177 have shown that hydrated electrons are produced with low efficiency upon light absorption by the lowest excited state of Ru(bpy) +, i.e., in a biphotonic process. 

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Finally, it is conceivable that a CTTL excited state can eject an electron into the solution medium. In systems where this process has been observed, however, it has proved difficult to decide whether the reactive state is CTTL or CTTS in character. A discussion of photoelectron production is presented in the following section. 

A characteristic reaction of CTTS excited states is photoelectron production with concomitant oxidation of the metal center.107 In the example given in equation (46),108 electrostatic repulsion of the primary photoproducts facilitates their separation and allows direct observation of the solvated electron. In other systems, photoelectron production has been inferred from the products observed in the presence of an electron scavenger such as N20 or CHC13.109... 

B. Photoelectron Production from Transition Metal Complexes. 93... 

Primary photoreactions leading to net oxidation or reduction reactions of coordination compounds are well known and are often the result of decay paths accessible only from CT states. A number of coordination compounds yield photoelectron production in solution, the Ru(2,2 -bipyridine)3+ ion has been shown to be an electron donor from its electronically excited state, and photoreduction of several metal complexes has been studied in detail. Discussion of these three areas should reveal most of the important principles associated with photoredox and CT state chemistry. 

Photoelectron production in solution has been verified or suspected from a number of metal complexes. A collection of metal ions that form complexes which likely yield photoelectrons upon irradiation includes Cr(II), Mo(IV), W(IV), Fe(II), and Ru(II).2 It is readily appreciated that a photoelectron producing species must have available an oxidation state one level above the starting level and one which is energetically accessible with light of optical energies. 

Among the several examples of photoelectron producing complexes, Fe(CN)4-has been studied in greatest detail.170-177) The quantum yield for photoelectron production is strongly wavelength dependent, Table 28,177 and shows definitively... 

Table 28. Wavelength dependence for photoelectron production from Fe(CN)g- )...

The eight-coordinate (fi complexes [Mo(CN)g]" and [W(CN)g] undergo two wavelength-dependent photoreactions. Photolysis in the short-wavelength (254 nm) region leads to efficient photoelectron production from an excited state assigned as CTTS in character. Irradiation... 

Figure 15.1. Schematic diagram illustrating the production of primary photoelectrons and Auger (secondary) electrons in an atom. In primary-photoelectron production, an atom absorbs an x-ray photon that causes the ejection of a core electron (in this case, a K-shell electron). An Auger electron is produced when, after the primary photoelectron has been ejected from the inner shell (indicated by the dotted lines), an electron from a higher shell (in this case, from the L shell) fills the orbital vacancy. The excess energy, instead of being emitted as a secondary x-ray, is simultaneously transferred to another electron (the Auger electron) which is ejected from the atom in this case the Auger electron is also from the L shell.

This distribution is illustrated in Fig. 3, for n = 2 and n = 20, which shows the evolution to a Gaussian distribution for large samples. For so-called photon-counting systems, where individual photoelectron events can be observed, such as in a photomultiplier, this random distribution may be observed directly in the presence of a constant optical power. In the more usual case, where the photoelectron production rate is much greater than the system... 

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