• Fundamental absorptions are vibrational transitions from the ground state to the first excited state. They give rise to the bands of greater intensity.
  • Overtones are vibrational transitions from the ground state to higher excited states. The absorption frequencies are $2/nu,3\nu...$ where $\nu$ is the frequency of the fundamental absorption.
  • Combination bands, result from the coupling of 2 bands $\nu_1$ and $\nu_2$ to give rise to a new frequency band $\nu_{comb}=\nu_1 + \nu_2$
  • Difference bands, , result from the coupling of 2 bands $\nu_1$ and $\nu_2$ to give rise to a new frequency band $\nu_{comb}=\nu_1 - \nu_2$
  • Fermi resonance, result from the coupling of a fundamental absorption band with an overtone or a combination band.