Photon echo due to Fermi-resonance vibrational levels of molecules

A nonlinear coherent optical effect is predicted: it is based on compensation of the Doppler phases of Fermiresonance vibrations of polyatomic molecules in the case of multifrequency laser excitation, resembling the phenomenon of the photon echo in gases. An analysis is made of the formation of radiation at the sum frequency in the case of two-frequency excitation of a combination vibration. It is shown that the new effect can be used for coherent speetroseopy of nonlinear intermode resonances.