Distant pairs

Fig. 4.18. The tunnelling luminescence kinetics for F, Vk pairs in KBr (upper curves 1, 2, 3) vs. temperature stimulation (lower curves 1, 2, 3 respectively), a is the slope change after the stimulation is switched off. According to the partial lightsum method [37, 87-90] the insert (curve 4) yields the relative dissimilar defect distribution occurring after the temperature stimulation. Arrows show the change of both the tunnelling luminescence decay intensity I(t) and the spatial defect distribution caused by the stimulation a close defect concentration increases whereas that for the distant pairs decreases.

Therefore, the method of partial lightsums illustrates once more effects of the defect diffusion a number of close pairs increases, whereas that of distant pairs decreases, unlike the case of defect rotation. 

Features The stem of this well-known wild plant is slender, square and hairy it gives off a few distant pairs of rough, oblong leaves with rounded teeth. Purplish flowers, arranged in a terminal, oval spike, bloom in July and August. The roots are white and thready. Bitter to the taste, the odour is slight and pleasant. 

At time t, all pairs for which R< R. have recombined, while the more distant pairs remain. From the shape of the distribution in Fig. 8.16, about 20 % of the pairs remain after 1 s, reducing to about 5 % after 1000 s, and this is consistent with the LESR data. The measurable range of the recombination over which the radiative tunneling mechanism applies is therefore more than 10 orders of magnitude, from 10" to 10 s. 

The reconciliation of these two apparently conflicting results is quite interesting and is connected to the broad distribution of recombination lifetimes. Those electron-hole pairs which are created with small separations are more likely to result in geminate recombination than the more distant pairs. The close pairs are also more likely to contribute to the luminescence and the distant pairs to LESR. Thus the two experiments are selectively measuring different parts of the distribution. The density of geminate pairs of separation, R, is given by. 

As the exponent k increases, the contributions of the most distant pairs of atoms become the most important. 

The STH resonance is enhanced in ODMR spectra by the addition of parts per million of known electron trapping dopants such as Ir3+ [176], Ni2 + [177], and Rh3+ [178]. This is probably because these dopants enhance distant pair formation at the expense of exciton formation [179,180]. Extrinsic self-trapped hole species have also been observed in the ODMR spectra of AgClj Br samples (see below) [111, 181,182],... 

EPR. This trend reflects the exchange interaction between the electron and hole. For given concentrations of donors and acceptors, there is a distribution in the separations between pairs. Slow microwave-modulation frequencies can emphasize the importance of the sharper, more distant pairs (see FIGURE 2). So far, the best donor linewidth of about 1 mT has precluded the observation of N-hyperfine splitting in the 3C polytype, which EPR has shown to be 0.12mT. 

A — attachment by two distant pairs of carbon atoms, with little departure from the planar zigzag chain. 

The tunneling probability depends on separation distance between D and A - the larger separation distance, the lower recombination probability, and longer lifetime of survived A and D. The closest DAPs recombine already during irradiation, providing PL. The more distant pairs deplete successively during... 

Complete linkage clustering The distance between two clusters A and B is defined as the distance between the most distant pair of objects, one from each cluster ... 

Kovalenko NP, Doycho IK, Gevelyuk SA, Vorobyeva VA, Roizin YO (1999) Geminate and distant-pair radiative recombination in porous silicon. J Phys Condens Matt ll(24) 4783-4800 Kovalev D, Heckler H, Ben-Chorin M, Polisski G, SchwartzkopffM, Koch F (1998) Breakdown of the k-conservation rule in Si nanocrystals. Phys Rev Lett 81(13) 2803-2806 Koyama H, Koshida N (1997) Spectroscopic analysis of the blue-green emission from oxidized porous silicon possible evidence for Si-nanostructure-based mechanisms. Solid State Commun 103(1) 37-41... 

The hydrodynamic interactions allow the distance between a nearby pair of beads to grow more rapidly compared with a distant pair. For the latter, the short-time mean square displacement increases as in the Rouse model. 

Approximation of Distant Pair Energies in Local MP2 Calculations. 

It is hoped that the quasi-crystalline water framework between electron donors and electron acceptors will also fixate electron transfer agents, e.g. tyrosine or other phenol derivatives. Alignment may be possible. The distant pairs of electron donors and acceptors proposed in section 6 can thus be connected and hopefully produce useful, non-covalent charge separation systems. 

Since Ar and AR are both positive, we conclude from Eq. (38) that PL-ODMR is opposite in sign to ADMR. In Section III.B.2 we determined that 8N > 0 for geminate pairs and 8N < 0 for distant pairs. Equation (38)... 

A useful relation can be written if we assume that Rp = 0. This assumption is quite justified because this transition is dipole-forbidden anyhow. We can then simplify Eq. (37) to get 8I/I = -Rp/XR for geminate pairs and 8I/I = AR/XR for distant pairs. The latter relation is particularly powerful, as 8N/N in ADMR for distant pairs is given by -(AR/XR). We therefore have for the distant pair model quite a useful relation between PL-ODMR and ADMR, namely. 

We therefore conclude that the reduction in the HE photoexcitation density is caused by enhanced recombination associated with the spin 1/2 high energy excitations themselves, with unthermalized spin polarization. This can be understood in the context of the distant pair model discussed in Section 11I.B.2. 

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