Abstract:
A study is made of the two-exciton absorption of light in molecular crystals at nonzero
temperatures. A unitary transformation is proposed that enables one to diagonalize
partly the energy operator of the crystal and investigate directly the cases of both strong
and weak exciton-phonon coupling. For the case of strong coupling, the temperature dependences
of the line intensity for double absorption and the energy distance between
lines of a permutational doublet are obtained. Allowance for the exciton-phonon interaction
also leads to the possible appearance of new lines. Formulas are obtained for the
coefficient of absorption of light by a crystal in the frequency range corresponding to the
excitation of two Frenkel excitons and phonons.