Exciton band shapes

Emission spectra at these points are shown in Figure 8.2d. The band shapes were independent of the excitation intensity from 0.1 to 2.0 nJ pulse . The spectrum of the anthracene crystal with vibronic structures is ascribed to the fluorescence originating from the free exdton in the crystalline phase [1, 2], while the broad emission spectra of the pyrene microcrystal centered at 470 nm and that of the perylene microcrystal centered at 605 nm are, respectively, ascribed to the self-trapped exciton in the crystalline phase of pyrene and that of the a-type perylene crystal. These spectra clearly show that the femtosecond NIR pulse can produce excited singlet states in these microcrystals. 

Aggregate units sometimes provide exciton bands in absorption and/or emission spectra that are characteristic of the mode of interaction of the constituent molecules 156-60]. That is, they depend upon the orientation of the molecules (i.e.. their transition dipoles) with respect to each other, the number of molecules interacting, and the distance between the chromophoric/lumophoric centers. The excitonic interactions result from the excited state of one molecule being affected by the electronic distribution of neighboring molecules. The location and shape of exciton bands can provide useful information concerning the relative orientations of molecules within an aggregate. 

The detection of what is believed to be optically detected signals for the zf transitions of triplet excitons has been recently reported (31). By fitting the observed band shape to a theoretical expression based on a one-dimensional model (63), a minimum coherent length... 

An experimental approach to measurement of the spatial coherence of excitons is offered by analysis of optical line shapes and hnewidths. However, a quantitative understanding of the exciton-phonon interaction is rendered difficult by the fact that the bandwidfhs of exciton bands are relatively small (in the singlet state lOOcm, in the triplet state 10cm ). The exciton-phonon interaction is not small in comparison to this. In the literature, one thus can find rather widely divergent numbers for coherence times which were measured by different methods. 

Fig. 8.4 Calculated absorption spectra of homodimers with four different geometries. The orientations of the transition dipoles and the geometrical factor k (Fig. (7.17)) are indicated in the box below each spectrum. The absorption spectrum of the monomer is shown with a dotted line in each panel and the spectrum of the dimer with a solid line. From left to right, the relative dipole strengths of the and Fs+ transitions are 0 2, 1 1, 0 2 and 0.5 1.5. ( b is the high-energy transition in A and the low-energy transition in C and D.) The sum of the dipole strengths is always twice the dipole strength of the monomer. The exciton bands have been given Gaussian shapes with an arbitrary width. See Fig. 9.7A for another illustration...

Exciton Absorption Band Shapes and Dynamic Localization of Excitations... 

Fig. 9.7 SHex contributes opposite rotational strengths to the exciton bands of a dimer. The dashed lines in (A) show the exciton absorption bands of a dimer the solid curve is the total absorption spectrum. In (B), the dashed lines are the circular dichroism (CD) of the two bands and the solid curve Is the total CD spectrum. The spectra are for a homodimer with = Di,a 2) =10 D, I/ 2iI = 7 A, 9 = 71°, a = P = 90° (Fig. 7.2) and ba = 4,444 A. This geometry makes H21 positive (7 21 = 50 cm in the point-dipole approximation) and gives the exciton band the higher transition energy, the larger dipole strength, and a positive rotational strength. For purposes of Illustration, the exciton bands were assigned Gaussian shapes with arbitrary widths...

Even in semiconductors, where it might appear that the exciton binding energies would be of interest only for low temperaPire regimes, excitonic effects can strongly alter tlie line shape of excitations away from the band gap. 

As the size of a semiconductor crystal becomes small a regime is entered in which the electronic properties, e.g. ionization potential and electron affinity, are determined by size and shape of the crystals [113], When a quantum of light (hv) with energy exceeding the band gap falls on the surface of a semiconductor crystal there appears a bounded electron-hole pair known as an exciton... 

Toyozawa Y. (1958) Theory of line-shapes of the exciton absorption bands. Progr. Theor. Phys. 20, 53-81. 

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