Photoexcited 4 capture

L. X. Chen, W. J. H. Jager, G. Jennings, D. J. Gosztola, A. Munkholm, and J. P. Messier, Capturing a photoexcited molecular structure through time-domain X-ray absorption hne structure. Science 292(5515), 262-264 (2001). 

The first step in a photoelectrochemical reaction is photon absorption (capture) by the substrate and the change in electron energy (photoexcitation) occurring in the substrate as a result of photon absorption. This step is followed by other chemical or electrochemical reaction steps involving the activated substrate. 

A shift of the flat band potential due to photoexcitation of the type shown in Fig. 10-18 results from the capture of holes in the surface state level, e , on the electrode as shown in Fig. 10-19. We now consider a dissolution reaction involving the anodic transfer of ions of a simple elemental semiconductor electrode according to Eqns. 10-24 and 10-25 ... 

Regardless of the nature of the surface state it is clear that it can capture an electron from the conduction band producing cathodic current. This cathodic current balances the anodic current produced when the photoexcited holes produced the oxidized surface state. The net result of these two processes is electron-hole recombination leading to no net current. This recombination process is what controls the voltage of photocurrent onset as can be seen in curve 2 of Figure 5. 

The results summarized here illustrate the important role surface states play in C>2 evolution from photoexcited TiC>2 and provide an example of a quantitative determination of the density and electron capture cross section of these states. 

Photoexcitation of n-type semiconductors renders the surface highly activated toward electron transfer reactions. Capture of the photogenerated oxidizing equivalent (hole) by an adsorbed oxidizable organic molecule initiates a redox sequence which ultimately produces unique oxidation products. Furthermore, specific one electron routes can be observed on such irradiated surfaces. The irradiated semiconductor employed as a single crystalline electrode, as an amorphous powder, or as an optically transparent colloid, thus acts as both a reaction template and as a directed electron acceptor. Recent examples from our laboratory will be presented to illustrate the control of oxidative cleavage reactions which can be achieved with these heterogeneous photocatalysts. 

Direct verification of DR-mechanism of DIET was provided [21] by combining the state-selective photoexcitation of the sample and the controlled thermally induced release of electrons from electron traps (Fig.9a). In RGS, after electron-hole pair creation at selective excitation by photons with energies E>Eg, the hole may survive and be self-trapped if the electron is captured by any kind of traps [32], In solid Ar at T>2 K the main part of electron traps is not active [12], the electron-hole recombination occurs before self-trapping the holes, and, therefore, the concentration of W-band emitting centers decreases (Fig.9a). On the contrary, the heating... 

In long-afterglow phosphors, optical excitation energy is stored in the lattice by trapping photoexcited charge carriers. The most prominent example is SrAl204 Eu,Dy after optical excitation of Eu, Eu is oxidized to Eu and Dy is reduced to Dy. Thermal excitation of Dy to Dy, followed by capture of the electron by Eu and subsequent Eu emission results in time-delayed Eu emission. The thermal excitation process of Dy determines the time delay. This particular material still generates visible emission after several hours in the dark. 

In our experiments the role of the [(PQ2+)n]8Urf. ls t0 rapidly capture the photoexcited electrons the Pt(0) or Pd(0) equilibrates the [(PQ2+/+)n]surf. with the (H2O/H2) couple. Overall, the result is the catalysis of the process represented by equation (10). All mechanisms for catalysis of this process... 

Whereas under photoexcitation the exciplex is excited indirectly via energy transfer from the excitons, it is the primary neutral excitation in electroluminescence. This is shown in Fig. 2.24, parts (a) and (b), where the EL emission for both TFB and PFB blends is dominated by the exciplexes. This becomes particularly clear when comparing the EL spectra with the delayed emission spectra in Fig. 2.23, parts (c) and (d). In contrast, the time-integrated PL from similarly prepared blend films (also plotted in Fig. 2.24) is primarily due to bulk excitons. We note that exciplex EL emission has been observed previously, which suggests that these exciplexes may also be formed by the mechanism of direct electron-hole capture at the interface [37, 41, 42]. 

The scheme in Figure 1 depicts an energy level diagram for energy capture by charge separation, in which a photoexcited molecule transfers an electron to a vacant orbital in a second molecule, which then transfers it to a third. 

Figure 8a illustrates two processes that lead to a decrease of the negative surface charge (1) direct optical excitation of trapped electrons into the conduction band and (2) capture of photoexcited holes from the valence band (Goldstein and Szostak, 1980). At high light intensities this produces a neutral surface and AFjj = V,. However, depending on the relative capture... 

Photoexcitation of the EDA complex of cis-27 or trans-27 and TCNE under an oxygen atmosphere also lead to formation of m-28 as the major product. In these cases, the trimethylene cation radicals formed from 27 are captured by molecular oxygen and the ensuing stepwise cyclization then yields 28. Interestingly, oxygenation to form 30 occurs when an acetonitrile solution of 29 is irradiated X > 390 nm) under an oxygen atmosphere in the presence of trifluoroacetic acid. Presumably, the carbenium ion 31 or 32 is initially formed from 29 and the photoexcited 31 or 32 then sensitizes oxygenation. 

The intense colors in 2,2 bipyridyl complexes of iron, ruthenium, and osmium are due to excitation of an electron from metal t2g orbitals to the empty jr -orbitals of the conjugated 2,2 bipyridyl. The photoexcitation of this MLCT excited state can lead to emission. However, not all complexes are luminescent because of the different competing deactivation pathways. This aspect is beyond the scope of this chapter the interested reader can refer to a number of publications on this subject [16-20]. The other potential deactivation pathways for the excited dye are donation of an electron (called oxidative quenching, Eq. 2) or the capture of an electron (reductive quenching, Eq. 3) or transfer of its energy to other molecules or... 

Figure 29.12 CMsBr moLecuLe physisorbed on an adatom site on Si (7 x 7). The dark arrow indicates the reaction site for the downward-propelled bromine atom the lighter arrow indicates the upward ejection of CH3. Both processes occur after capture of a photoexcited substrate electron see text for details. Adapted from Osgood et al, Surface Science, 2004, 573 147, with permission of Elsevier...

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