Broadening exciton-phonon coupling

In Section I, the spectra of e"(ai) consist of Dirac 5 peaks (1.79). In a real crystal these peaks are broadened by static disorder, thermal fluctuations, and excitation-relaxation processes. Discarding for the moment the static disorder, we focus our attention on broadening processes due to lattice phonons, which may be described alternatively in terms of fluctuations of the local energies of the sites, or in terms of exciton relaxation by emission and absorption of phonons. These two complementary aspects of the fluctuation-dissipation theorem64 will allow us to treat the exciton-phonon coupling in the so-called strong and weak cases. The extraordinary (polariton) 0-0 transition of the anthracene crystal will be analyzed on the basis of these theoretical considerations and the semiexperimental data of the Kramers-Kronig analysis. 

Figs. 2.9,2.10, recorded at very low temperatures and in high resolution. For a crystal of very high quality, the broadening of these transitions is mainly due to phone- >. We briefly analyze their influence on the reflectivity in order to assure that no spurious structures are considered. These spectra have been recorded for the first time and analyzed by Turlet et al.1-67. We simply summarize a few points necessary to test the KK transformation, to point out the specificity of the intrinsic relaxation mechanisms related to exciton-phonon couplings, and to evaluate quantitatively the corresponding coupling parameters. 

We found a correct account of the broadening magnitude and increase with temperature, out of reach of the renormalized perturbation theory. Exact numerical calculations by Schreiber and Toyozawa53 agree with our data and confirm our values of the exciton-phonon coupling strengths. 

Figure 3.13. Temperature evolution of the parameter re T) of the first-surface exciton (hollow circles). The full circles indicate the values obtained for the bulk exciton (Fig. 2.14). The quasi-coincidence at T 0 is fortuitous, but the surface states appear less broadened than the bulk states in the region 30-50 K, which could correspond to a decay at the surface of the density of interplane phonons coupled to the exciton (see Section III.B).

In addition to the photoluminescence red shifts, broadening of photoluminescence spectra and decrease in the photoluminescence quantum efficiency are reported with increasing temperature. The spectral broadening is due to scattering by coupling of excitons with acoustic and LO phonons [22]. The decrease in the photoluminescence quantum efficiency is due to non-radiative relaxation from the thermally activated state. The Stark effect also produces photoluminescence spectral shifts in CdSe quantum dots [23]. Large red shifts up to 75 meV are reported in the photoluminescence spectra of CdSe quantum dots under an applied electric field of 350 kVcm . Here, the applied electric field decreases or cancels a component in the excited state dipole that is parallel to the applied field the excited state dipole is contributed by the charge carriers present on the surface of the quantum dots. 

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