Quantum beat spectral distribution

Either two or more molecular levels of a molecule are excited coherently by a spectrally broad, short laser pulse (level-crossing and quantum-beat spectroscopy) or a whole ensemble of many atoms or molecules is coherently excited simultaneously into identical levels (photon-echo spectroscopy). This coherent excitation alters the spatial distribution or the time dependence of the total, emitted, or absorbed radiation amplitude, when compared with incoherent excitation. Whereas methods of incoherent spectroscopy measure only the total intensity, which is proportional to the population density and therefore to the square ir of the wave function iff, the coherent techniques, on the other hand, yield additional information on the amplitudes and phases of ir. 

The Fourier transform of the time-resolved fluorescence intensity I(t) yields its spectral distribution I(o)) with sub-Doppler resolution. Figure 11.24 illustrates as an example quantum beats measured by ANDRA et al. [11.19] in the fluorescence following the excitation of three hfs levels in the 6p P3/2 state of the ion. Either a tunable dye laser crossed perpen-... 

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