Consistent Paradigm of the Spectra Decomposition into Independent Resonance Lines

The shapes of the spin resonance spectra have been analyzed theoretically in the case, when the kinetic equation for the spin density matrix is linear. Examples of the random relaxation processes which lead to the linear kinetic equations for the spin coherences have been presented in short. A consistent approach has been described for the decomposition of the multicomponent spectra into individual resonance lines based on finding independent collective modes for the evolution of quantum coherences. For the model situations with two and three transitions between the energy levels, the spectra are decomposed following this approach. The contributions of their collective evolution modes to the magnetic resonance spectra have been analyzed comprehensively. In the presence of the coherence transfer, the shapes of resonance lines corresponding to these modes can be a mixture of Lorentzian absorption and dispersion curves. The results obtained make it possible to visualize in detail transformations of the spectra as a consequence of the coherence transfer caused by random relaxation processes. This consistent approach makes it possible to describe on a common platform the transformations of spectra at any coherence transfer rate: from the very slow coherence transfer rate which leads to line broadening, then to the rate which results in the coalescence of the spectral lines and further to the very fast rate which leads to exchange narrowed spectra. It is shown that in the limit of the fast coherence transfer corresponding to the exchange narrowing effect one collective mode gives the dominant contribution to the experimental spectrum, namely, in-phase evolution of all transition coherences. The shape of the resonance corresponding to this in-phase evolution is described by the narrowed Lorentzian absorption curve.