Exciton-polaron

This is the operator (or matrix) equation for the coherent potential v(z). The formal expression for v(z) can be easily written. The exciton polaron is... 

The energy spectrum of the exciton polaron as a function of k is obtained by the solution of an equation... 

Such renormalization can be obtained in the framework of the small polaron theory [3]. Scoq is the energy gain of exciton localization. Let us note that the condition (20) and, therefore, Eq.(26) is correct for S 5/wo and arbitrary B/ujq for the lowest energy of the exciton polaron. So Eq.(26) can be used to evaluate the energy of a self-trapped exciton when the energy of the vibrational or lattice relaxation is much larger then the exciton bandwidth. 

In Science, every concept, question, conclusion, experimental result, method, theory or relationship is always open to reexamination. Molecules do exist Nevertheless, there are serious questions about precise definition. Some of these questions lie at the foundations of modem physics, and some involve states of aggregation or extreme conditions such as intense radiation fields or the region of the continuum. There are some molecular properties that are definable only within limits, for example, the geometrical stmcture of non-rigid molecules, properties consistent with the uncertainty principle, or those limited by the negleet of quantum-field, relativistic or other effects. And there are properties which depend specifically on a state of aggregation, such as superconductivity, ferroelectric (and anti), ferromagnetic (and anti), superfluidity, excitons. polarons, etc. Thus, any molecular definition may need to be extended in a more complex situation. 

Mobile defects are Frenkel78 excitons, Mott-Wannier excitons, polarons, bipolarons, polaritons, and solitons. 

K. Timpmann, M. Ratsep, C.N. Hunter, A. Freiberg, Emitting excitonic polaron states in Core LHl and peripheral LH2 bacterial light-harvesting complexes. J. Phys. Chem. B 108, 10581-10588 (2004)... 

This concept of polarons and, in particular, excitonic polarons has been used to explain observed features of the one-dimensional conducting organic materials based on 7,7,8,8-tetracyano-p-quinodimethane, TCNQ (85). It indicates that a way to reduce the Coulomb repulsion between electrons in the chain is to surround each chain by a highly polarizable medium. However, a limit may be reached beyond which, if the surrounding medium were made more polari zable, the effects due to band narrowing would outweigh the benefits of reduced Coulomb repulsion (85). 

The working mechanism of the device is shown schematically in Figure 5.3, where the levels of the electrodes and the HOMO (highest occupied molecular orbital) and LUMO (lowest rmoccupied molecular orbital) of the polymers are reported, together with the intragap levels of the polarons and the exciton-polaron. The position of the HOMO and... 

Many theories have been developed (involving solitons, excitons, polarons and bipolarons) [17] to explain the conductivity phenomenon under the assumption that the chains of conductive polymers are being arranged and at least somewhat oriented in fibrils. But now, it must be explained why our dispersed (and later flocculated) polymer showed principally the same transport properties as the fibrillar conductive polymers, as can be concluded from conductivity versus temperature and thermopower measurements. 

Since the excited states are electrically neutral, only the short-range, acoustic interaction is relevant in eqn (7.24). (This is also true for the exciton-polaron, as the particle and hole are closely separated.) The polaron, however, being charged also couples to the longitudinal optic phonons, so the long-range term is retained... 

A useful simplification to this expression arises by noting that the geometric distortions of the polarons and exciton polarons (namely the or states) from the ground state structure are similar (as described in Chapter 7). Thus, the Huang-Rhys parameter (proportional to AQi, as defined in Fig. 9.14) for the 1 B and 1 E states relative to the positive polaron is negligible. Therefore, to a good approximation,... 

In contrast, the exciton-polaron (l B. )) state is expected to be in the adiabatic limit, as its energy in the undimerized Pariser-Parr-Pople model is 1.6 eV in the long chain limit. This is confirmed by Fig. 10.7, which show the deviations between the quantum and adiabatic limits is only ca. 0.1 eV at 102 sites. 

The l Bj state is now an exciton-polaron. Its structure is qualitatively similar in both the noninteracting and interacting limits, as the soliton-antisoliton confinement due to linear confinement arising from the effective extrinsic bond alternation has a rather similar effect to electron-hole attraction. However, as already predicted, the state has a more pronounced distortion because it... 

Keywords Photosynthesis, purple bacteria, excitons, polarons, ultrafast dynamics, conducting polymers, semiconductor films. 

Buckminsterfullerene (C50) has appeared to be an efficient photosensitizer for the PPV derivatives (61a) and (6ab). Ultrafast (> 1 ps) photoinduced electron transfer from PPV derivatives to C6o results in complete chaige separation, quenching of photoluminescence and enhancement of the photoconductivity by several orders of magnitude [429,435]. The photo-excited exciton-polaron is speculated to migrate along 50-100 monomer units of the polymer chain during its lifetime and to dissociate on encountering C o [429]. 

Recently the theoretical models developed for polyacetylene have been applied to PDAs (00, 115, 116). A degenerate ground state does not occur for PDA so that this discussion is limited to polarons. The potential surface for the ground state structures deduced by this method is very similar to that calculated by the ab initio method (00, 106) indicating that the alternative butatriene structure is metastable. A further conclusion is that the band gap is primarily due to the bond alternation produced when the TT-electrons are added to the bare PDA backbone. These calculations are currently being extended to make realistic exciton calculations for PDAs (117). The occurrence of acousticmode polarons and exciton-polarons in PDAs has also been discussed recently (95, 145). 

Brazovskii, S. A. and Kirova, N. (1981) "Excitons, Polarons, and Bipolarons in Conducting Polymers" Sov. Phys. JEPT Lett. 33, 4-8. 

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