Optical centers, interaction with states

On the other hand, at point B (optical branch), the two atoms move in opposite directions, but the center of gravity of the unit cell remains unshifted (Fig. 1.46b). Furthermore, the equivalent atom in each lattice moves in phase. If the two atoms carry opposite electrical charges, such a motion produces an oscillating dipole moment that can interact with incident IR radiation. Thus, it is possible to observe it optically. It should be noted that the frequency of a diatomic molecule in the free state is CO = V/Tf, whereas that of a diatomic lattice is co = j2f (p = reduced mass). 

One well-known example of the physical system where the described approach can be applied is the Aj-E optical transition in trigonal centers with the E-e-type Jahn-Teller interaction. In this case the AP in the electronic E-state has a shape of the Mexican hat [11 -14]. If the E-e-type Jahn-Teller interaction is strong, then the Mexican hat is large and the AP has a long minimum line along the trough. In this... 

A suitable choice of the variational wave functions for various electron-phonon two-level systems is a long-standing problem in solid state physics as well as in quantum optics. For two-level reflection symmetric systems with intralevel electron-phonon interaction the approach with a variational two-center squeezed coherent phonon wave function was found to yield the lowest ground state energy. The two-center wave function was constructed as a linear combination of the phonon wave functions related to both levels introducing new VP. 

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