Excitons coherent

Section IV is devoted to excitons in a disordered lattice. In the first subsection, restricted to the 2D radiant exciton, we study how the coherent emission is hampered by such disorder as thermal fluctuation, static disorder, or surface annihilation by surface-molecule photodimerization. A sharp transition is shown to take place between coherent emission at low temperature (or weak extended disorder) and incoherent emission of small excitonic coherence domains at high temperature (strong extended disorder). Whereas a mean-field theory correctly deals with the long-range forces involved in emission, these approximations are reviewed and tested on a simple model case the nondipolar triplet naphthalene exciton. The very strong disorder then makes the inclusion of aggregates in the theory compulsory. From all this study, our conclusion is that an effective-medium theory needs an effective interaction as well as an effective potential, as shown by the comparison of our theoretical results with exact numerical calculations, with very satisfactory agreement at all concentrations. Lastly, the 3D case of a dipolar exciton with disorder is discussed qualitatively. 

This section has been devoted to the study of the surface excitons of the (001) face of the anthracene crystal, which behave as 2D perturbed excitons. They have been analyzed in reflectivity and transmission spectra, as well as in excitation spectra bf the first surface fluorescence. The theoretical study in Section III.A of a perfect isolated layer of dipoles explains one of the most important characteristics of the 2D surface excitons their abnormally strong radiative width of about 15 cm -1, corresponding to an emission power 10s to 106 times stronger than that of the isolated molecule. Also, the dominant excitonic coherence means that the intrinsic properties of the crystal can be used readily in the analysis of the spectroscopy of high-quality crystals any nonradiative phenomena of the crystal imperfections are residual or can be treated validly as perturbations. The main phenomena are accounted for by the excitons and phonons of the perfect crystal, their mutual interactions, and their coupling to the internal and external radiation induced by the crystal symmetry. No ad hoc parameters are necessary to account for the observed structures. 

In the last two sections we analyzed spectral and relaxation properties of 3D and 2D strong dipolar excitons in high-quality crystals at low temperatures in terms of the strong excitonic coherence of band width 500 cm l, preserving the properties of the quasi-ideal crystal structure (what we called the intrinsic surface-bulk system) in the presence of weak disorder A... 

In these expressions the Liouville space Green function matrix elements representing one-exciton coherence are... 

Lee H, Cheng Y-C, Fleming GR. Coherence dynamics in photosynthesis protein protection of excitonic coherence. Science 2007 316 1462-1465. 

Exciton coherence. Payer, M.D.in Spectroscopy and Excitation Dynamics of Condensed Molecular Systems. Agranovich,V.M. Hochstrasser, R.M. Eds. North-Holland Amsterdam, 1983, p.l85. 

Figure 6. (A) Scaling and saturation of the lowest three nonvanishing static polarizabilities (a, y, and c) of polyacetylene chains with size (B) variation of the scaling exponents b = d[ln)( ]ld[lnN, % = a, y, s with size for the curves shown in panel A. Note how the exciton coherence size increases with the degree of nonlinearity. Reprinted with permission from ref 89. Copyright 1996 Elsevier Science.

Meier, T., Chernyak, V., Mukamel, S. Multiple exciton coherence sizes in photosynthetic antenna complexes viewed by pump-probe spectroscopy. J. Phys. Chem. B 101, 7332-7342 (1997)... 

Exciton coherence , M.D. Payer in Spectroscopy and excitation dynamics of condensed molecular systems . Eds., V.M. Agranovich and R.M. Hochstrasser, North-Holland, Amsterdam, 1983, p. 185. 

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