Fermi resonance is a phenomenon that occurs in molecular spectroscopy when two vibrational modes of a molecule become coupled due to their interaction, particularly when they have similar energies. This coupling leads to a shift in the energy levels of the vibrational modes, resulting in a mixing of their characteristics. In situations of Fermi resonance, one vibrational mode, typically a fundamental vibration, can influence another mode—often an overtone or a combination band—with which it is energetically close.
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