Centers with trapped holes

Centers with trapped holes. For instance, bands due to CI2 on anion lattice site absorbing at the high-frequency side of the F-band are called H- and V-bands. 

The second important effect is that irradiation absorption generates active states of the photoadsorption centers with trapped electrons and holes. By definition (Serpone and Emeline, 2002) the photoadsorption center is a surface site which reaches an active state after photoexcitation and then it is able to form photoadsorbed species by chemical interaction with substrate (molecules, or atoms, or ions) at solid/fluid interface. In turn, the active state of a surface photoadsorption center is an electronically excited surface center, i.e. surface defect with trapped photogenerated charge carrier that interacts with atoms, molecules or ions at the solid/gas or solidfiquid interfaces with formation of chemisorbed species. ... 

Since the original studies of F centers many other color centers have been characterized that may be associated with either trapped electrons or trapped holes. These are called electron excess centers when electrons are trapped and hole excess centers when holes are trapped. 

The color center is the [A104]4 group, which can be thought of as [A104]5 together with a trapped hole. The color arises when the trapped hole absorbs radiation. 

In this connection it should be remembered that we can distinguish between recombination centers and traps. On the one hand, electron or hole traps can be considered as states from which trapped electrons or holes will with relatively high probability be reexcited thermally into the conduction or valence band. 

A description of the emission and capture processes at a trap will be useful before discussing the various experimental methods. Figure 1 depicts the capture and emission processes that can occur at a center with electron energy ET. The subscripts n and p denote electron and hole transitions, and the superscripts t and differentiate between thermally and optically stimulated processes. It is assumed here that only thermal capture processes are occurring. 

Fig. 14. Some Auger processes involving one-free carrier (boles as illustrated) The case of two trapped electrons on the same center is shown in (a), and the situation for trapping on nearby centers is shown in (b). The case of an exciton (isoelec-tronic) type center, with electron recombination to the trapped hole is shown in (c), and recombination with a free hole in (d) [note that in practice these two processes have to be considered in parallel (see, for example, Neumark, 1973)].

Complementary studies of neutral [20] and charged [16] intrinsic trapped centers, comparison of cathodoluminescence [21] and thermoluminescence data [12] with results of analysis of photoelectron scattering [13] and pump-probe experiments [14] allow us to extend the energy relaxation scheme (Fig.2d, dotted arrows) including electron-hole recombination channels. The formation of H-band emitting centers (R2+) occurs through the excitation of STH by an exciton. The bulk recombination of trapped holes with electrons populates the (R2 ) states with subsequent M-band emission [22], After surface recombination of STH with electrons the excited dimers escape from the surface of the crystal with subsequent IF-band emission. 

Formation of Ti3+ centers in Ti02 from trapped electrons is usually connected with generation of various radicals from trapped holes, but such reactions and species will be discussed in section 8.3. 

If the two 0 ions are coupled, the dimer species formed can be expected to exist either as a ground-state singlet or triplet, depending on its nature. In the alkaline-earth oxides, bulk V° centers have been reported these centers consist of a cation vacancy with two hole centers, one trapped either side of the vacancy. Their spectroscopic behavior is characterized as an axial 5 = 1 center and can be described by the spin Hamiltonian,... 

The results for the bulk centers in the alkaline-earth oxides show that, in principle, EPR spectra could be obtained for a pair of O" ions separated by R on the surface. However, there are no examples of well-resolved fine-structure spectra for such adsorbed species. The main reasons for this are expected to be as follows From the above formula, one would anticipate the spacings between the lines in the parallel and parpendicular directions to increase with decreasing distance R. It is therefore likely that with the heterogeneity of sites that has been discussed previously for isolated O species, the EPR spectrum would be difficult to observe. Electrostatic repulsion will favor the collinear orientation of the two trapped holes on opposite sides of the positive ion vacancy, as observed from the relaxation... 

Two Cr ions (represented by large circles) move together, with a hole, to form a CI2 ion. This self-trapped hole , also called a V, center, is immobile. 

Figure 5 shows TL spectra of the sample ZASM. Before irradiation by mercury lamp, no peaks are found in TL spectra of the sample while after irradiation, three broad peaks are induced in TL spectra of the samples which are treated at round 99°C, 178°C and 302°C, respectively. The results imply that there are at least involved with three kinds of electron or hole trap centers with different trap depth in the ceramic, which finally leads to the LLP. 

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