Exci tons

The recombination rate Ptlx, which is defined as the number of generated exci-tons in A/ divided by the number of generated excitons in At plus the number of minority carriers passing through the device in At, for an unbalanced LED (where j(x) jp(x) 3> j (x) and pp pH) is given by the following equation ... 

In more recent studies, Jacobs, Tompkins and Young [136] examined the rate of evolution of nitrogen from barium azide as a criterion of the rate of photolysis, and have shown the reaction to be more complex than was previously indicated. A mechanism for the photolysis involving the production and reaction of both exci-tons and positive holes has been formulated. 

A short, intermediate-level review of the structure and function of the light-harvesting complex of the purple bacteria and exci-ton flow to the reaction center. 

Besides charge transfer interactions, dipolar coupling between ttk transitions of bases may lead to delocalization of the excited states. In order to obtain some guidelines for our experimental studies, we have undertaken the calculation of excited Frank-Condon states within the framework of the exci-ton theory [26]. These studies were enriched by combining data from quantum chemistry and molecular dynamics calculations in collaboration with Krystyna Zakrzewska and Richard Lavery [26,27,27-29]. The general formalism is described in the Chapter by E. Bittner and A. Czader in the present volume. 

The amplitude parameter selection rules for the discrete and continuum exci-ton contributions were found concordant with those of the Eq transitions. 

Quintet states on a single molecule require the simultaneous excitation of two electrons and are, therefore, energetically inaccessible (at least in aromatics in the solid phase). This is the reason for which the quintet states in the final products of reactions (80) and (81) have been omitted. Any of the nine pair states is formed with equal probability since the individual triplet exci-tons are in thermal equilibrium. Thus the rate of formation of each of the nine l states is (1/9) K, and K 1 Cg, and C T are the rate transitions to energetically accessible final... 

If 4>no and (A = 1,2) are localized ground- and excited-state wave functions of the chromophores k, the ground state of the two-chromophore system may be described by Pq = 4>,o4>2o. whereas the excited states 4 = N (4>i,4>2o 4> 4>2f) are degenerate in zero-order approximation. The exci-ton-chirality model only takes into account the interaction between the transition dipole moments A/, and localized in the chromophores. Thus, the interaction gives rise to a Davydov splitting by 2Vj2 of the energies of combinations and of locally excited states. From the dipole-dipole approximation one obtains... 

At high excitation densities in the solid state, the decay of the singlet exciton becomes excitation dependant, bimolecular annihilation of the singlet exci-tons introduces a fast component to the decay [41,42,77,78], this is shown in Fig. 21. In a number of publications pump-probe spectroscopy has been used to study the phenomena surrounding this accelerated decay. The bimolecular annihilation reaction is effectively energy transfer from one excited singlet to... 

In earlier studies in these laboratories it hes been shown that polymers containing repeating naphthalene or phenanthrene groups and small numbers (from 0.1 to 2%) of traps such ets anthracene, anthraquinone, or jAienyl ketone, demonstrated singlet exci-ton transfer from the absorbing site in the antenna to the trap [1]. The efficiency of energy transfer in the trap can be evaluated in terms of the quantity x, vrtiich is defined as the number of photons transferred to the trap divided by the number of photons absorbed by the antenna. 

The commonly observed larger scale of phase separation of the toluene cast MDMO-PPV PCBM blends has generally been interpreted as the main reason for the reduced photocurrents in comparison to those of the chlorobenzene cast blends. It can then be expected that a lower charge carrier generation efficiency may result when exciton diffusion lengths of 10-20 nm are well exceeded by the PCBM cluster size (200-500 nm), since many exci-tons are generated within these clusters. Experimentally, it has been identified that indeed some unquenched photoexcitations give rise to residual PCBM photoluminescence in toluene cast blends, whereas in chlorobenzene cast... 

It was mentioned in Sect. 1.3.2 that in semiconductors, isoelectronic impurity centres could present a relatively strong attracting potential for electrons or holes. Excitons can be trapped by or created at these isoelectronic centres to form an isoelectronic bound exciton (IBE). The electron (hole) of this exci-ton is also more strongly bound to the isoelectronic centre than in classical excitons and the second constituent of the exciton, hole (electron) can be considered to be bound to a compound negative or positive ion. These structures are similar to those of neutral donors or acceptors and they are called isoelectronic donors or acceptors [104]. When formed by near band-gap or above band-gap laser illumination, the long lifetimes of these IBEs result in sharp PL lines, and this has for some time aroused interest for these centres as potential near IR radiation emitters. 

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