Transitions, molecular crystals, spectra

The PPV spectra of Fig. 16 show all the signatures of exciton absorption and emission, such as in typical molecular crystals. The existence of well-defined structure in the absorption spectrum is not so easily accounted for in a band-to-band absorption model. In semiconductor theory, the main source of structure is in the joint density of states, and none is predicted in one-dimensional band structure calculations (see below). However, CPs have high-energy phonons (molecular vibrations) which are known (see, e.g., RRS spectra) to be coupled to the electron states. The influence of these vibrations has not been included in previous theories of band-to-band transition spectra in the case of such wide bands [176,183]. For excitons, the vibronic structure is washed out in the case of very intense transitions, corresponding to very wide exciton bands, the strong-coupling case [168,170]. Does a similar effect occur for one-electron bands Further theoretical work would be useful. 

Crystals grown from the melt necessarily have internal strains, and their dislocation densities can easily be in the range of 10 to 10 dislocations/cm. The difference is made clear in Fig. 4.5. The 0,0 line of the Ti <- So transition in an anthracene crystal grown by the Bridgman procedure shows more inhomogeneous broadening by a large factor than that of a sublimation platelet. There, the linewidth is only 0.009 cm Fig. 4.5 makes it very clear that the sublimation crystals are vastly superior in terms of the number of defects in the crystal lattice. Here, we already refer to the characteristic features in the optical spectrum of molecular crystals. These will be treated in more detail in Chap. 6. 

In the following 20 years, a group of physicists in the Ukraine [2] studied a series of other aromatic crystals spectroscopically. It developed that there are also very characteristic differences from the spectra of free molecules. In the year 1948, A. S. Davydov [3] formulated the basic theoretical explanation for the observable interaction processes in the crystal spectra, between the molecules in electronically excited states within the crystal. He made use of the model of Frenkel excitons [4] and was able in particular to give a quantitative explanation of a characteristic line splitting, the Davydov splitting, as a fundamental property of organic molecular crystals. Fig. 6.1 shows as an example the splitting of the 0,0-transition in the Ti So absorption spectrum of anthracene at room temperature. 

Chapter 7 introduces the reader to solutions of many selected problems in molecular physics. In particular, the following important problems are studied in detail the fluorescence spectrum ofp-terphenyl crystal, the vibrational fine structure of the spin-allowed absorption band of rans-[Co(CN)2(f )2]Cl3H20, and transport phenomena of electronic excitation in pentacene-doped molecular crystals. It is followed by an analysis of phosphorescence and radiationless transition in aromatic molecules with nonbonding electrons as well as predissociation of the 82 state of H2O+ by nonadiabatic interaction via conical intersection. 

There are a few exceptions to the statements of the previous paragraph. The vibrational Raman spectrum of liquid H2 shows rotational fine structure for H2, the rotational levels are widely spaced and intermolecular forces are reasonably small. Certain solids when heated undergo a transition to a solid state in which molecular rotation in the crystal is possible. Solid H2 undergoes such a transition, as shown by the heat-capacity curve see Davidsoriy Section 16-9. 

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