Excitonic molecule

PL evidence for the binding of more than one exciton to a shallow impurity exists, starting with the excitonic molecule was first reported in silicon [20]. A model for the bound multi-exciton complexes in silicon (the shell model) has been elaborated by Kirczenow [27] to explain the experimental results of these centres. For a review on these centres, see [49]. 

The problem of electronic biexcitons (excitonic molecules) in molecular crystals has been repeatedly discussed (see Ref. (97) and references therein). Such attention is motivated by the important role which, as in the case of semiconductors... 

In this case we must consider the collective properties of biexctons, and not of excitons. The possibility of formation of biexcitons (excitonic molecules) has been indicated in (10). They have been found experimentally by Haynes (11). 

The AC Stark effect is relevant, not only in atomic spectroscopy, but also in solid state physics. The biexciton state (or excitonic molecule), where two Wannier excitons are bound by the exchange interaction between electrons, occurs in various semiconductors (see section 2.22). Various experiments on the AC Stark effect of excitons have been reported, but the clearest example to date is probably the observation of the Rabi splitting of the biexciton line in CuC reported by Shimano and Kuwata-Gonokami [477]. It is very interesting to consider how Bloch states in solids, which themselves are delocalised and periodic, are dressed or modified by the electromagnetic field, since their properties are rather different from those of purely atomic states, which are by definition completely localised. 

For the very unusual bandshape of the lowest transition band (Fig. 11) different interpretations were offered. By applying the excitonic molecule model [146] (a two-dimensional dipole picture), Oelkrug and his coworkers pointed first to an excitonic band structure [64, 67] with a width of 10000cm" where the ft = 0 states correspond to the highest energy levels within the excitonic band, whereas the bottom of the band is located at the zone boundary with ft 5 0. The apparent blue-shift in the spectra would then correspond to the width of the excitonic band. A very similar interpretation was published later by Kanemitsu et al. [147], again neglecting the... 

The SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

Bach FI, Renn A, Zumofen G and Wild U P 1999 Exciton dynamics probed by single molecule spectroscopy Phys. Rev. Lett. 82 2195-8... 

Let us now consider the case where tliere is more tlian one exciton in tlie given molecular ensemble. The presence of two or more excess excitons not only creates two or more holes in tlie ground state (see case (a) above) but it also opens up tlie possibility of two excitons being found on neighbouring molecules. Then tlie following two-stage process can take place [26] ... 

In tlie first stage, where at first we have two excitons S, excitation jumps from one of tlie excited molecules to... 

In order to demonstrate the NDCPA a model of a system of excitons strongly coupled to phonons in a crystal with one molecule per unit cell is chosen. This model is called here the molecular crystal model. The Hamiltonian of... 

Here ak a ) is the annihilation (creation) operator of an exciton with the momentum k and energy Ek, operator an(a ) annihilates (creates) an exciton at the n-th site, 6,(6lt,) is the annihilation (creation) operator of a phonon with the momentum q and energy u) q), x q) is the exciton-phonon coupling function, N is the total number of crystal molecules. The exciton energy is Ek = fo + tfcj where eo is the change of the energy of a crystal molecule with excitation, and tk is the Fourier transform of the energy transfer matrix elements. 

To verify effectiveness of NDCPA we carried out the calculations of absorption spectra for a system of excitons locally and linearly coupled to Einstein phonons at zero temperature in cubic crystal with one molecule per unit cell (probably the simplest model of exciton-phonon system of organic crystals). Absorption spectrum is defined as an imaginary part of one-exciton Green s function taken at zero value of exciton momentum vector... 

The picture presented above for confinement of the excitons within the device is for the EM layer sandwiched between the HTL and ETL. The EM need not be a discrete layer in the OLED, however, for exciton confinement to occur. Alternatively, the EM can consist of a luminescent molecule doped (- 1%) into a polymeric or molecular host material (40,41,54,55). So long as the energy gap (or band gap) of the host is higher than that of the EM dopant, excitons will be effectively trapped or confined on the dopant molecules leading to improved EL efficiency. An example of such a dopant-based device... 

C. Resonance energy transfer. The excitation energy can be transferred by resonance energy transfer, a radiationless process, to a neighboring molecule if their energy level difference corresponds to the quantum of excitation energy. In this process, the quantum, or so-called exciton, is transferred. 

The nature of the light emissions is influenced by the way in which the absorbed energy is transferred through the polymer matrix. In crystalline polymers, exciton migration is possible as all molecules lose their energetic individuality and all electronic and oscillation levels are coupled [20]. Thus, new exciton absorption and emission bands are formed and the excitation energy can move along the chain ... 

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