Vibronic complexes, influence

Because the energy state of a Jahn-Teller complex depends on the local lattice distortions, the macroscopic long-distant strain that produces an ultrasonic wave should influence it as well. The cross effect is initiated by the Jahn-Teller complexes (1) the dispersion (i.e., frequency-dependent variation of phase velocity) and (2) attenuation of the wave. In terms of the elastic moduli it sounds as appearance (or account) of the Jahn-Teller contribution to the real and imaginary parts of the elastic moduli. For a small-amplitude wave it is a summand Ac. Obviously, interaction between the Jahn-Teller system and the ultrasonic wave takes place only if the wave, while its propagation in a crystal, produces the lattice distortions corresponding to one of the vibronic modes. 

We shall begin our discussion with the derivation of line shape functions for optically allowed and vibronically induced transitions. This will permit us to demonstrate the influence of the Duschinsky rotation on optical spectra. As an example, we shall describe the spectrum of the p-terphenyl crystal from the same point of view and of the [Co(CN)2(tn)2]Cl3H20 complex in its crystal structure. These examples offer little more than a glimpse into the difficult, subtle, fascinating questions encountered in almost any attempt to interpret optical transitions. The discussion is extended in Section 7.2 to the analysis of phosphorescence spectra and the description of radiationless transition of aromatic molecules. Particular emphasis is placed on the mechanism of singlet-triplet relaxation in these molecules with nonbonding electrons. 

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