Solvated electrons from photolysis

Because the formation rate of solvated electrons from DHS photolysis is extremely low, it was considered to be relevant only for the elimination of highly refractive compounds. 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

The formation of the transient species Ag° by reduction of Ag+ from hydrated electrons using a double flash photolysis technique has been observed.199 This species may be photodissociated at 315 nm, probably via charge-transfer to solvent, to produce Ag+ and solvated electrons. 

Photoreductiom in Aqueous Solution. Solvated electrons form on the irradiation of natural water samples or of aqueous solutions of natural organic matter isolated from surface waters (12, 45, 46). The solvated electron is a powerful reductant that reacts rapidly with electronegative substances such as chlorinated, brominated, and iodinated compounds. The interaction of pho-toejected electrons and a halocarbon is demonstrated by the laser flash photolysis data shown in Figure 3. 

Trichloroacetate rapidly reacts with the solvated electrons produced by laser flash photolysis of natural organic matter isolated from the Suwannee River, and thus quenches the absorption of the electrons at 720 nm. The ibsorption is also quenched by the addition of other good electron acceptors, including oxygen, protons, or nitrous oxide. In natural waters, halocarbon concentrations are typically very low, and the dominant scavenger of solvated electrons is oxygen. 

Figure 3. Effects of trichloroacetate (0.0050 M) on the absorption (720 nm) of solvated electrons produced by laser flash photolysis (355 nm) of argon-saturated solutions (pH 6.2) of natural organic matter isolated from the Suwannee River near Fargo, Georgia (12). A, no added trichloroacetate B, with added...

Photolysis. Absorption of UV or visible light from a flash lamp or a laser is used to produce the solvated electron.Two processes may occur (1) the photon energy is sufficient to ionise a solute and produce the electron, for instance, photo-detachment from an anion (Cl", Fe(CN)g ", Na"...), (2) the laser intensity is high enough to induce ionisation of the solvent by multiphoton absorption (e.g. H O -1- nhv HjO " + e"). 

The data from flash photolysis experiments (266-nm excitation) on naproxen in aqueous solution at pH 7 are shown in Figure 2.10. The transient spectra were identified as due to the solvated electron (absorbing at 700 nm) and a triplet state (absorbing at 430 nm). Similar species were found for sulfamethoxazole. Under this pH condition, both molecules are present as the anion, so photoionization is a likely process with the high energy excitation. The solvated electron generation implied a radical production so EPR with spin trapping was performed. 

Other Studies Using Flash Photolysis. Flash photolysis allows the study of direct electron capture (17). A flash of light may eject an electron from A ",Cat+, for example. The primary products are a solvated electron (e ) and an adduct (A,Na+). The adduct may dissociate before electron capture and lead to the following reaction ... 

On irradiation of DOM, the one primary photochemical event is photo-ionization. This process is less sensitive to environmental factors than triplet formation. All of the remaining photochemistry depends on the subsequent fate of the three primary products of photoionization triplets, solvated electrons, and cation radicals [13]. Laser flash photolysis studies supplied early evidence for hydrated electron (e q) production on irradiation of DOM, with significant primary quantum yields [9,12,84,85]. The hydrated electron was identified by its characteristic broad absorption spectrum from 700 nm to 750 nm [86, 87]. The formation of e q is thought to result from the photo-ejection of an electron from excited-state humic substances ... 

Flash photolysis and electron-pulse techniques may be considered as cases of extreme perturbing functions, in the first case an extremely intense flash of light, in the other case a beam of electrons. Such perturbations cause extreme deviations from equilibrium concentrations, so that linear first-order rate equations no longer describe the time behavior of the system. In fact, molecules are often promoted to higher electronic states. With the advent of the laser, the time resolution of flash photolysis has been reduced to picoseconds. This permits the study of processes not previously accessible to kineticists. The types of systems studied include radiationless transitions, the solvated electron and chlorophyll. For example, the cage effect in liquids has been demonstrated by a study of the recombination of iodine atoms at very short times [8]. Although these methods are of considerable interest, we do not discuss them in further detail here (cf. Hammes [1]). 

A simplified view of the early processes in electron solvation is given in Fig. 9 (i) the electron is ejected from a molecule upon ionization by radiolysis or photolysis (ii) in the thermalization step, the ejected electron progressively loses its excess kinetic energy in collisions with solvent molecules (iii) then, the electron is localized, trapped in a solvent site or cavity and (iv) becomes solvated when the solvent molecules have obtained their equilibrium configuration after relaxation. 

---