Translation-Invariant Excitons in a Phonon Field

Large-radius excitons in polar crystals are considered. It is shown that translation invariant description of interacting with a phonon field leads to nonzero contribution phonons into the exciton ground state energy only case weak or intermediate electron-phonon coupling. A conclusion made self-trapped cannot exist limit strong Peculiarities absorption and emission spectra discussed. Conditions when hydrogen-like model remains valid interaction found.