Atomic inversion after a photon detection

Spectral detection of resonance fluorescence allows to regard atomic spectra as a sequence of elementary scattering events accompanied by emissions of photons with different frequencies. What the reduced atomic state (RAS) is after such a photon detection represents fundamental interest and appears important in understanding some correlation experiments. We report about derivation of analytic expression for the RAS for the important particular case of the well separated spectral lines of the resonance fluorescence spectrum.