Photoexcited electrons

Another common loss process results from electron—hole recombination. In this process, the photoexcited electron in the LUMO falls back into the HOMO rather than transferring into the conduction band. This inefficiency can be mitigated by using supersensitizing molecules which donate an electron to the HOMO of the excited sensitizing dye, thereby precluding electron—hole recombination. In optimally sensitized commercial products, dyes... 

Fig. 5.14 Inoue et al. carried out a systematic study of the photocatalytic reduction of CO2 by different semiconductor powders in aqueous suspensions. Shown here is the energy correlation between semiconductor catalysts and redox couples in water, as presented in their paper. In principle, the solution species with more positive redox potential with respect to the conduction band level of the semiconductor is preferably reduced at the electrode. Photoexcited electrons in the more negative conduction band certainly have greater ability to reduce CO2 in the solution. (Reproduced from [240])...

Theory Ti02 acts as photocatalyst due to generation of photoexcited electrons and holes which were involved in decomposition of KI. Under ultrasonic irradiation efficiency of iodine release increased almost linearly with irradiation time. 

Summary. Coherent optical phonons are the lattice atoms vibrating in phase with each other over a macroscopic spatial region. With sub-10 fs laser pulses, one can impulsively excite the coherent phonons of a frequency up to 50THz, and detect them optically as a periodic modulation of electric susceptibility. The generation and relaxation processes depend critically on the coupling of the phonon mode to photoexcited electrons. Real-time observation of coherent phonons can thus offer crucial insight into the dynamic nature of the coupling, especially in extremely nonequilibrium conditions under intense photoexcitation. 

The photoexcited electrons are trapped at the surface by TiIV sites, subsequent to this process resulting Tim sites, which may further transfer charge carriers to molecular oxygen resulting in the formation of a superoxide radical ... 

J. F. Letard, R. Lapouyade, and W. Rettig, Relaxation pathways in photoexcited electron-rich stilbenes (D-D stilbenes) as compared to D-A stilbenes, Chem. Phys. Lett. 222, 209-216 (1994). 

Under light illumination, semiconductor electrodes absorb the energy of photons to produce excited electrons and holes in the conduction and valence bands. Compared with photoelectrons in metals, photoexcited electrons and holes in semiconductors are relatively stable so that the photo-effect on electrode reactions manifests itself more distinctly with semiconductor electrodes than with metal electrodes. 

In the case in which the photoexcited pairs of electrons and holes are relatively stable so that thermal equilibrium is established between phonons and electrons in the conduction band as well as between phonons and holes in the valence band, we can define individually the electrodiemical potentials of photoexcited electrons and of holes in the photostationary state. Here, thermal equilibrium is not established between the photoexcited electrons in the conduction band and the holes in the valence band. The electrochemical potential, thus defined, for the photoexcited electrons and holes is caUed the quasi-Fermi level of electrons nCp, and the quasi-Fermi level of holes,[Schockley, 1950 Gerischer, 1990]. 

Since photoexcited electron-hole pairs are formed only within a limited depth from the semiconductor surface to which the irradiating photons can penetrate, the photon-induced split of the Fermi level into the quasi-Fermi levels of electrons and holes occurs only in a surface layer of limited depth as shown in Fig. 10-2. 

Photoexcited electrons or holes migrate in a space charge layer towards the electrode interface, where they participate in transfer reactions of cathodic electrons or anodic holes to provide a reaction current as shown in Pig. 10-9. Such a reaction current of photoexcited electrons or holes is called the photoexcited reaction current or simply the photocurrent. 

As shown in Fig. 10-9, the photoexcited reaction current occurs only when an appreciable electric field exists in the space chai ge layer. No photocurrent occurs at the flat band potential because no electric field that is required to separate the photoexcited electron-hole pairs is present. The photocurrent occurs at any potentials different from the flat band potential hence, the flat band potential may be regarded as the potential for the onset of the photocurrent. It follows, then, that photoexcited electrode reactions may occur at potentials at which the same electrode reactions are thermodynamically impossible in the dark. 

The rate of the formation of photoexcited electron-hole pairs, Gix), is given as a function of the intensity of photon beam h, the absorption coefiicient of photons a, and the depth of photon-penetration x as shown in Eqn. 10-12 [Butler, 1977] ... 

The point at which the straight line of (tph) versus Eintersects the coordinate of electrode potential represents the flat band potential. Equation 10-15 holds when the reaction rate at the electrode interface is much greater than the rate of the formation of photoexcited electron-liole pairs here, the interfadal reaction is in the state of quasi-equilibrium and the interfadal overvoltage t)j, is dose to zero. 

Similarly, the cathodic photoexcited electron transfer of the h3 rogen reaction shown in Eqn. 10-19 can occur at p-type semiconductor electrodes at which the cathodic hydrogen reaction is thermodynamically impossible in the dark. 

In photoexcited n-type semiconductor electrodes, photoexcited electron-hole pairs recombine in the electrodes in addition to the transfer of holes or electrons across the electrode interface. The recombination of photoexcited holes with electrons in the space charge layer requires a cathodic electron flow from the electrode interior towards the electrode interface. The current associated with the recombination of cathodic holes, im, in n-type electrodes, at which the interfadal reaction is in equilibrium, has already been given by Eqn. 8-70. Assuming that Eqn. 8-70 applies not only to equilibrium but also to non-equilibrium transfer reactions involving interfadal holes, we obtain Eqn. 10-43 ... 

One way of experimentally exploring the electronic structure of solids is by means of photoemission spectroscopies such as UPS and X-ray photoelectron spectroscopy (XPS), where photoexcited electrons are analyzed dispersively as a function of their kinetic energy. The electronic structure of the reference material TTF-TCNQ will be extensively discussed in Section 6.1. Figure 1.31 shows the XPS spectra of the S2p core line for (TMTTF)2PF6 (black dots) and BEDT-TTF (grey dots). 

Cyclic photophosphorylation in purple bacteria. QH2 is eventually dehydrogenated in the cytochrome bc1 complex, and the electrons can be returned to the reaction center by the small soluble cytochrome c2, where it reduces the bound tetraheme cytochrome or reacts directly with the special pair in Rhodobacter spheroides. The overall reaction provides for a cyclic photophosphorylation (Fig. 23-32) that pumps 3-4 H+ across the membrane into the periplasmic space utilizing the energy of the two photoexcited electrons. 

Figure 4.11 explains how the loaded metal can act as catalyst for both the reductive and oxidative reactions. The essential point is that the barrier height at the metal/semiconductor interface is changeable by the principle of Fig. 4 10 or other mechanisms. Thus, when the band bending is weak, photoexcited electrons mostly enter the metal and the metal acts as a catalyst for a reductive reaction (Fig. 4.11(A)). On the other hand, when the band bending is strong, the holes in the valence band mostly enter the metal and the metal acts as a catalyst for an oxidative reaction (Fig. 4.11(B)). The prevalence of one over the other depends on the magnitude of the band bending, i.e., on the relative rates of reaction of the electrons and holes at the metal-free semiconductor surface.

---