Strong excitonic coherence

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... 

Coherent optical phonons can couple with localized excitations such as excitons and defect centers. For example, strong exciton-phonon coupling was demonstrated for lead phtalocyanine (PbPc) [79] and Cul [80] as an intense enhancement of the coherent phonon amplitude at the excitonic resonances. In alkali halides [81-83], nuclear wave-packets localized near F centers were observed as periodic modulations of the luminescence spectra. 

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. 

Time and temperature dependences of the delayed fluorescence in isotopi-cally mixed naphthalene crystals have been presented for various concentrations of traps. Coherent two-photon processes in naphthalene in the strong exciton-photon counting regime have also been investigated. Excited-state spectra of 1,5-naphthyridine in several solvents support those calculated using INDO molecular orbital formalism and show the lowest excited singlet state to... 

In the strong coupling case, the transfer of excitation energy is faster than the nuclear vibrations and the vibrational relaxation ( 10 12 s). The excitation energy is not localized on one of the molecules but is truly delocalized over the two components (or more in multi-chromophoric systems). The transfer of excitation is a coherent process9 the excitation oscillates back and forth between D and A and is never more than instantaneously localized on either molecule. Such a delocalization is described in the frame of the exciton theory10 . 

The chl-chl coupling estimated seems to be somewhat at variance with the suggestion of very strong coupling in LHCII, leading to delocalized, coherent excitonic interactions [51, 168]. 

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