For the same reason, difference bands cannot be intensified by Fermi resonance.

Fermi resonance does not really lead to additional bands in the spectrum.

This phenomenon, known as Fermi resonance, occurs because the two vibrationally excited states mix.

Fermi resonance most often occurs between normal and overtone modes, if they are nearly coincident in energy.

Fermi resonance leads to two effects.

Remember that Fermi resonance is only possible when a fundamental and a second-order band have the same symmetry and are close together in energy.

Fermi resonance thus introduces additional strong bands, which may be mistaken for fundamentals; it also affects the apparent frequencies of fundamentals involved.

A Fermi resonance is the shifting of the energies and intensities of absorption bands in an infrared or Raman spectrum.

The phenomenon of Fermi resonance can arise when two modes are similar in energy; Fermi resonance results in an unexpected shift in energy and intensity of the bands etc.

The task of assignment may be complicated by the absence of some expected bands with fortuitously low intensity, or by the presence of formally-forbidden overtone or combination bands, which may have appreciable intensity if the conditions for Fermi resonance are satisfied.

Hot bands, as mentioned earlier, show the effects of anharmonicity in the formation of sequences of bands; as they are already formally allowed transitions there is no enhancement in intensity due to Fermi resonance, though mixing of upper-state levels may affect the regularity of hot-band sequences.

This coupling between group modes of the same symmetry should not be confused with Fermi resonance between a fundamental and an overtone or combination of the same symmetry [see Section 5.8.6 above]; in the case of coupling between fundamental group modes there is no violation of the vibrational selection rule.