Overtone band

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An overtone band is a weak feature in a molecule’s vibrational spectrum that occurs when the molecule moves from the ground vibrational state (v=0) to a higher excited state (v=2). If bonds were perfect harmonic oscillators, only Δv = ±1 transitions would be allowed, so no overtone bands would appear. Real bonds are anharmonic and are better described by a Morse potential, which allows transitions with Δv = ±2, ±3, and so on. The overtone here means the Δv = 2 transition (v=0 to v=2). In infrared spectra, overtone bands exist but are much weaker than the fundamental Δv = 1 band, which explains their low intensity.