Doppler spectroscopy of zero-phonon lines

 The small width of purely electronic zero-phonon lines (ZPL) opens attractive possibilities of their study with the help of Doppler effect (DS-Doppler Spectroscopy). In order to shift the ZPL with 10 MHz width to a new position the velocities of the detector relative to the source of the order of tens of meters per second are sufficient. The narrower the line or the spectral hole (its negative image), the more efficient is DS. The DS of the spectral holes shapes and the possibilities of the super-sensitive DS are discussed; the peculiarity of the DS frequency modulation (the perturbations of the internal states of the emitter and absorber are absent) is pointed out. Thus it is interesting to study the interference of the photon “with itself” when the frequency of “its half” is changed by DS.