The ground-singlet state of an optical bipolaron in the
region of intermediate coupling strengths is analysed.
It is shown
that the spatial structure of a bipolaron with intermediate
coupling strength is that of an axially symmetric quasi-molecular
dimer.
The bipolaron energies of coupling are determined for different coupling
constants and dielectric parameters of a medium.
The electronic terms of the ground-dimer state are represented as a
function of the distance between quasi-particles.
The intermediate
region of coupling constants is shown to give rise to an
additional specific term in the total energy,
which depends on the dielectric properties of a medium and makes
the bipolaron energy of coupling higher (at
ε*/ε∞>1.06) or lower (at
ε*/ε∞<1.06) than the value in the
limit of adiabatic and strong coupling.
The method of canonical transformation of coordinates is formulated for a strong-coupled system of electron and phonon field, taking into account the rigorous fulfillment of the conservation laws. The system of electronically excited terms of a bipolaron has been established. The sequence of the excited terms and their dependence on the distance between polarons have been obtained.
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