Relation to the Observed Spectrum Resonances

Although it is not our purpose to review this subject here, one aspect of this step in the calculation must be mentioned because of its importance in determining the anharmonic force field and its relationship to the contact 

The complication in analysing the spectrum is that the energy levels, and hence the spectral lines, can no longer be fitted by a power series in the quantum numbers that converges sufficiently rapidly to form a simple pattern of energy levels and spectral lines, so that simple analytical formulae cannot be used, and assignment becomes a problem. Usually the spectrum can only be analysed in conjunction with an appropriate theoretical treatment of the hamiltonian for the interacting levels. 

Despite the complication which resonances introduce into the analysis of a spectrum and the theoretical treatment of the hamiltonian, when they can be analysed they often give valuable information on the force field which cannot be obtained directly in the absence of a resonance. We consider briefly the two commonest types of resonance interaction, Fermi (or anharmonic) resonance and Coriolis resonance, to illustrate this point. 

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