Very shallow traps

Very Shallow Traps. It has been proposed that the neutral Gua(Nl—H) radical, formed by proton transfer from the Gua radical by proton transfer from N1 of Gua to N3 of Cyt, is a shallow trap [143,144]. This proposal is based on projections from made on monomers in dilute aqueous solution, which predict that proton transfer is favored by 2.3 kJ/mol [22,145]. Ab initio calculations are in excellent agreement with this value [146,147]. So one expects that an energy of at least 0.025 eV is needed to activate the return of the proton to N1 Gua, reforming Gua . Once Gua is reformed, tunneling to nearby guanines is reestablished as a competitive pathway. Proton transfer therefore is a gate for hole transfer. Proton-coupled hole transfer describes the thermally driven transfer of holes from one Gua Cyt base pair to another. 

The proposal that holes are detrapped at lower temperatures than the excess electrons is based on the observations discussed above in Very Shallow Traps and Shallow Traps (Sec. 4.3.2). One expects that the activation energy needed to detrap the hole from Gua in duplex DNA is relatively small, an order of magnitude less than that needed to detrap the electron. This fits well with the observation that upon warming 4 K irradiated crystalline DNA to 77 K, 10-30% of the radicals anneal out, i.e., at least one of the trapping sites [fide infra Gua(N3-H) ] is very shallow. 

The excitation moves by EET in the antenna system until it reaches the SP, which acts as a very shallow trap. Charge separation takes place within the RC, probably due to the slightly smaller distance between the chromophores. Subsequently, the electron moves to Pheoi via the accessory Chip,. It seems that the ETs are only on one side (Dl) of the dimeric RC, just as in the bacterial RC. 

Discussion of this example will serve to bring out some important consequences of this detailed examination of the diffusion process and may help to make its physical meaning more apparent. To recapitulate, Eq. (15) applies to a case depicted in Fig. 2, where an atom in going from one normally occupied site to a neighbor and must pass through two interstitial positions. If the stopping points 1 and 2 were very shallow traps or actually unstable positions, one could have the case where 0oi and 032 were equal and very much smaller than any of the other jump frequencies. For the moment take the other frequencies, 0j2, 021, 023 and 0,0, as equal, and it will follow that... 

Two types of fluorescence spectra are observed in aqueous solution and in the presence of an excess of cadmium ions. In the absence of any stabilizer the fluorescence is characterized by two very weak bands (41), one centered at 450 nm and attributed to the direct recombination of charge carriers (61) from shallow traps, and the other very broad at about 650 nm, which is not clearly attributed. The presence of a stabilizing agent, such as HMP (59,60), makes it possible to increase the sulfide vacancies at the surface resulting in a more intense fluorescence band, centered in the region of 550 nm. This band is attributed to the recombination of... 

Time-resolved measurements of photogenerated (very intense illumination, up to 0.56 GW/cm ) electron/hole recombination on CD (selenosulphate/NTA bath) CdSe of different crystal sizes has shown that the trapping of electrons, probably in surface states, occurs in ca. 0.5 ps, and a combination of (intensity-dependent) Auger recombination and shallow-trapped recombination occurs in a time frame of ca. 50 ps. A much slower (not measured) decay due to deeply trapped charges also occurred [102]. A different time-resolved photoluminescence study on similar films attributed emission to recombination from localized states [103]. In particular, the large difference in luminescence efficiency and lifetime between samples annealed in air and in vacuum evidenced the surface nature of these states. 

Of the various types of surface states available, evidence will be presented below which indicates that the only deep surface traps are adsorption traps. The possibility exists that some very shallow surface... 

We mentioned the main models for generation, transfer, and recombination of the charge carriers in polymers. Very often, these models are interwoven. For example, the photogeneration can be considered in the frame of the exciton model and transport in the frame of the hopping one. The concrete nature of the impurity centers, deep and shallow traps, intermediate neutral and charged states are specific for different types of polymers. We will try to take into account these perculiarities for different classes of the macro-molecules materials in the next sections. 

Sulfates. The basic lattice of sulfate phosphors absorbs very short wavelength UV radiation. On excitation with X rays or radiation from radioactive elements, a large proportion of the energy is stored in deep traps. For this reason, CaS04 Mn is used in solid-state dosimeters. Of the glowpeaks which can be selected by thermoluminescence, more than 50 % fail to appear at room temperature because of a self selection of the shallow traps. Other activators, such as lead or rare-earth ions (Dy3 +, Tm3 +, Sm3+), stabilize the trapped electrons [5.399]—[5.401]. 

A variety of processes can occur in the interaction of 02 molecules and Ag(l 11). At first scattering from and trapping in the physisorption potential can occur. Secondly, scattering from the chemisorption (02 ) potential occurs, together with transient trapping-desorption. The chemisorption potential well is very shallow. From being transiently trapped the molecule can be captured in the molecular chemisorption well presumably surface imperfections are necessary to stabilize the molecular adsorbate in this case. From the molecular chemisorption well the molecule can proceed to dissociation. In this step ad atoms may be involved on Ag(l 11). Finally, there is a small probability for direct dissociative chemisorption of 02 at Ag(l 11). The formation of added Ag-O rows (fences) at the surface inhibit further sticking at the surface. 

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