Overtone band
In vibrational spectroscopy, an overtone band is the spectral band that occurs in a vibrational spectrum of a molecule when the molecule makes a transition from the ground state (v=0) to the second excited state (v=2), where v is the vibrational quantum number (a non-negative integer) obtained from solving the Schrödinger equation for the molecule. Generally, in order to study the vibrational spectra of molecules, chemical bond vibrations are assumed to be approximable as simple harmonic oscillators.