Light energy quanta

Fig. 37. Potential dependences of current in dark and under illumination for polycrystalline diamond electrode in 0.1 M KH2PO4 solution. Light quantum energy (a) 6.4 eV (b) 5 eV. Incident power density 80 mW cm 2 [163]. Reproduced by permission of The Electrochemical Society, Inc.

The quantity Wph can be found from the dependence of the photoemission current on the light quantum energy by extrapolating it to the zero current. Reference 27 reports the work function as 5.95 eV for photoemission from aqueous solutions of FeCCN) ". Using the standard potential of the system Fe(CN)6 "/Fe(CN)6 as +0.36 V (relative to the normal hydrogen electrode), we obtain for the reorganization energy of water around ferrocyanide/fer-ricyanide ions 5.95 - 4.4 - 0.36 — 1.2 eV. For the system considered, the electron transfer reaction is an outer-sphere one, so one can take = R,red = Er. 

Irradiation of a semiconductor with light of quantum energy greater than the band gap can lead to electron-hole separation. This can affect adsorption and lead to photocatalyzed or photoassisted reactions [187]. See Section XVIII-9F for some specifics. 

When a hydrogen atom absorbs light, the energy of the photon converts it from the ground state to an excited state. In this process, its electron transfers to an orbital that has a higher energy and a larger principal quantum number. 

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... 

The dismutation (disproportioning) of two free radicals is accompanied by release of a portion of reaction energy as a light quantum. As the quantum yield of such a process is extremely low, the detection of this type of chemiluminescence is technically complicated. Several compounds like lucigenin and luminol have a high quantum yield after reaction with peroxide radicals. Therefore, they are widely used for the detection of these radicals, particularly in the examination of phagocyting cells. 

An active medium, consisting of a collection of atoms, molecnles, or ions in a gaseous, liquid, or solid state, which generates and amplifies light by means of appropriate transitions between its quantum energy levels. 

Q What is the quantum energy of visible light of wavelength 500 nm ... 

Experimental observation of photoemission currents encounters the problem of separating them from the currents of photoelectrochemical reactions of nonemission nature, which are caused by the internal photoeffect in the semiconductor (see, for example, Section 5). Photoprocesses of both the types start similarly with the interband excitation of an electron and are of threshold character with respect to the frequency of light, but the threshold quantum energy is different for these processes. Namely, the threshold of photoemission exceeds that of the internal photoeffect (and hence the threshold of ordinary photoelectrochemical reactions) by the value of the electron affinity to the semiconductor % (see Figs. 31 and 32). 

Fig. 33. Three-halves power law dependence of the photoemission current to the power 2/3 on the energy of light quantum for a given potential (a) and on the potential for a given energy of light quantum (b). p- type gallium arsenide (/VA = 6 x 1018 cm-3) in aqueous solution. [From Boikova el al. (1976).]...

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

An electron torn away from a molecule, M, of a matrix (or an additive) by a y-quantum, by a secondary electron, or by a light quantum [reaction (1)1 is thermalized, i.e. slowed down to the rate of thermal motion [process (2)] and is then captured by a trap T [reaction (3)]. For the electron to be stabilized in the trap the energy level in this trap should be lower than the botton of the matrix conduction band (Fig. 1). The experimental investigations carried... 

It has been noted by Potasek [105] that electron tunneling in the donor-acceptor pair D-A may lead to the appearance of a charge transfer band in the absorption spectrum of this pair. The author obtained the following formula describing the dependence of the extinction coefficient, , of this band on the energy, E, of the absorbed light quantum... 

The dialkyl peroxides absorb light below about 300 nm the quantum energy at this wavelength corresponds to 95 kcal mole-1, and since these compounds have activation energies for thermal homolysis on the order of 35 kcal mole-1 for the dialkyl peroxides and 30 kcal mole-1 for the diacyl peroxides,... 

---