High Quality Content by WIKIPEDIA articles! A molecular vibration occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant translational and rotational motion. The frequency of the periodic motion is known as a vibration frequency. A nonlinear molecule with n atoms has 3n?6 normal modes of vibration, whereas a linear molecule has 3n?5 normal modes of vibration because rotation about its molecular axis is simply a rotation of the reference frame and cannot be observed. A diatomic molecule thus has only one normal mode of vibration. The normal modes of vibration of polyatomic molecules are independent of each other, each involving simultaneous vibrations of different parts of the molecule. A molecular vibration is excited when the molecule absorbs a quantum of energy, E, corresponding to the vibration's frequency, ?, according to the relation E=h?, where h is Planck's constant. A fundamental vibration is excited when one such quantum of energy is absorbed by the molecule in its ground state. When two quanta are absorbed the first overtone is excited, and so on to higher overtones.

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