Intrinsic excitons

In an attack on the particular problem of excitons which are weakly bound to localized "Impurities", Rashba and Gurgenishvill (2) derived the following relation between the oscillator strength of the bound exciton and the oscillator strength of the Intrinsic excitons f, using the effective-mass approximation... 

It has been shown In some materials that F, exceeds f by more chan four orders of magnitude. An Inspection or Equation reveals Chat, as the Intrinsic exciton becomes more tightly bound to the associated center, the oscillator strength, and hence the Intensity of Che exciton complex line, should decrease as (l/E). ... 

They found good agreement between experiment and theory for surface-modified CdSe nanocrystallites. This agreement suggests that the band edge luminescence of CdSe QDs can be quantitatively understood using an intrinsic exciton model and that the role of the surface in the optical properties of the QDs lies in its defects which create deep red radiative and nonra-diative pathways for recombination of the electron and hole upon photoexcitation of the nanocrystallite [6]. 

B valence band, and lower F7 for C valence band) will result in the following intrinsic exciton ground-state symmetries ... 

S-S annihilation phenomena can be considered as a powerful tool for investigating tire exciton dynamics in molecular complexes [26]. However, in systems where tliat is not tire objective it can be a complication one would prefer to avoid. To tliis end, a measure of suitably conservative excitation conditions is to have tire parameter a< )T < 0.01. Here x is tire effective rate of intrinsic energy dissipation in tire ensemble if tire excitation is by CW light, and T = IS tire... 

In conclusion we stress once more that the above-considered mechanism of the effect of illumination on the adsorptivity and catalytic activity of a semiconductor holds in the case when the absorption of light increases the number of free electrons or holes (or both) in the crystal. This, however, does not always take place. The absorption of light by the crystal may proceed by an exciton mechanism. This seems to be the case in the region of intrinsic absorption, which is as a rule photoelectrically inactive. 

Fig. 2 Schematic representation of charge-carrier generation via singlet excitons. (a) Singlet/electrode (b) singlet-singlet (c) intrinsic...

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. 

Various models are used in the literature to account for the kinetics of the excitons involved in optical processes. In the simplest cases, the signal evolution n(t) can be reproduced by considering either a single exponential or multiexponential time dependences. This model is well suited for solutions or solids in which monomolecular mechanisms happen alone. Since in most transient experiments the temporal response is a convolution of a Gaussian-shaped pulse and of the intrinsic kinetics, the rate of change with time of the excited-state population decaying exponentially is given by... 

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