Vibronic excitons Davydov splitting

To first order, we consider the molecular structure of the surface layers to be identical to that of the bulk layers. Consequently, all the characteristics corresponding to short-range intralayer interactions (e.g. Davydov splitting, vibrational frequencies, excitonic band structure, vibronic relaxations are similar for bulk and surface layers). In fact, we shall see that even slight changes may be detected. They will be analyzed in Section III.C, devoted to surface reconstruction. Therefore, our crystal model consists of (a,b) monolayers translated in energy relative to the bulk excitation by 206, 10, and 2cm-1 for the first three layers, as indicated in Fig. 3.5. No other changes are considered in this first-order crystal model. 

In triplet transitions, the resonance interaction and thus the Davydov splitting can, as mentioned, no longer be understood in terms of a dipole interaction, but rather as an exchange betv een orbitals which overlap. Measured values are available only for Ti states. Figure 6.10 shows as an example, similar to Fig. 6.1, the two 0,0 components of the excitation spectrum Ti So, of the triplet exciton Ti in an anthracene crystal here, however, the spectrum was taken at 1.8 K. 0,0 transition means that the excitation affects only excitons and that no additional phonons or vibrons are excited. In anthracene, the Davydov splitting of the Ti excitonic state is 21.5 cm" (cf Fig. 6.10). In naphthalene, it is 9.8 cm [34]. 

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