Defect centers

Using a variety of transient and CW spectroscopies spanning the time domains from ps to ms, we have identified the dominant intrachain photoexcitations in C )-doped PPV films. These are spin-correlated polaron pairs, which are formed within picoseconds following exciton diffusion and subsequent dissociation at photoinduced PPV+/Cw> defect centers. We found that the higher-energy PA band of polaron pairs is blue-shifted by about 0.4 eV compared to that of isolated polarons in PPV. 

A series of calculations on defect centers induced by radiation damage in alpha-quartz is reported. Ab initio SCF-MO calculations were carried out on a 21 atom cluster, Si50i6 % surrounded by 956 point-ions, designed to simulate alpha-quartz. This two-region approach made it possible to represent the long-range electrostatic effects, present in the crystal, in the SCF-MO cluster. 

The requirement I > 2 can be understood from the symmetry considerations. The case of no restoring force, 1=1, corresponds to a domain translation. Within our picture, this mode corresponds to the tunneling transition itself. The translation of the defects center of mass violates momentum conservation and thus must be accompanied by absorbing a phonon. Such resonant processes couple linearly to the lattice strain and contribute the most to the phonon absorption at the low temperatures, dominated by one-phonon processes. On the other hand, I = 0 corresponds to a uniform dilation of the shell. This mode is formally related to the domain growth at T>Tg and is described by the theory in Xia and Wolynes [ 1 ]. It is thus possible, in principle, to interpret our formalism as a multipole expansion of the interaction of the domain with the rest of the sample. Harmonics with I > 2 correspond to pure shape modulations of the membrane. 

In Chapter 4, Corbett deals with specific defect centers in semiconductors. He points out that H aids the motion of dislocations in Si, which can lead to enbrittlement. Throughout this chapter, Corbett raises many questions that need further exploration. For example Is oxygen involved in processes that are attributed to hydrogen Does H play a role in defect formation ... 

Electrical studies show that H neutralizes defects at relatively low temperatures, suggesting that H is very mobile at low temperatures. Corbett points out various defect centers in III-V materials that appear to be affected by H. 

Coherent optical phonons can couple with localized excitations such as excitons and defect centers. For example, strong exciton-phonon coupling was demonstrated for lead phtalocyanine (PbPc) [79] and Cul [80] as an intense enhancement of the coherent phonon amplitude at the excitonic resonances. In alkali halides [81-83], nuclear wave-packets localized near F centers were observed as periodic modulations of the luminescence spectra. 

H. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohki, K. Nagasawa, and Y. Hama, Photoluminescence from defect centers in high-purity silica glasses observed under 7.9-eV excitation, Phys. Rw. B 45, 586-591 (1992). 

Note 3 Schlieren textures observed in nematic samples with planar alignment show defect centers with two or four emerging brushes. Schlieren textures in nematic samples with tilted alignments show centers with four brushes centers with two brushes are caused by defect walls. 

Minerals of tin are capable of intrinsic luminescence, possibly connected with defect centers containing Sn T The ionic radius of Sn" " is of 0.83 A and the possible substituting luminescence center is Ti" " with an ionic radius of 0.75 A. 

Electronic conduction in crystalline semiconductors (except for the case of extremely high doping levels or very low temperatures) invariably involves motion in extended states. However, because of the high densities of defect centers, the possibility exists for transport by direct tunneling between localized states. 

In this chapter, the effect of preexcitation with the light of band-gap energy on trapping and thermal generation is examined in selenium and selenium-rich As-Se alloy films by several techniques. Results suggest that excess carrier trapping and dark-carrier generation are controlled by deep defect centers whose population can temporarily be altered by photoexcitation. 

Luminescence of Lattice Defects. Many defect centers are known in the case of the alkali-metal halides, which are derived from electrons in anion vacancies (F-centers, or color centers). Association of two or more F-centers gives new defect centers, which can each also take up an electron. These lattice defects act as luminescence centers, the emission spectra of which sometimes exhibit a large number of lines. 

Tynan and Yen (1969) have suggested that the association of aromatic sheets in the asphaltene macrostructure may occur through coordination of heterocycles. The aromatic sheets may have defect centers with heteroelements providing coordination centers for metals (Yen, 1974). 

In semiconductor phosphors the energy band structure of the host crystal plays a central role. Some semiconductor luminescence arises from decay of exciton states, other emission arises from decay of donor states generated by impurity or defect centers. It is not the magnitude of the band gap itself that separates insulator from semiconductor phosphors it is a question of whether the spectrum is characteristic of impurity energy levels as perturbed by the local crystal structure or whether the spectrum is characteristic of the band structure as modified by impurities. 

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