Self-trapped Exciton and Franck-Condon Spectra Predicted in LaMnO${}_3$

Because the ground state has cooperative Jahn-Teller order, electronic excitations in LaMnO${}_3$ are predicted to self-trap by local rearrangement of the lattice. The optical spectrum should show a Franck-Condon series, that is, a Gaussian envelope of vibrational sidebands. Existing data are reinterpreted in this way. The Raman spectrum is predicted to have strong multiphonon features.