[roonie1988]

hi, could someone please explain what does "anharmonicity" means in vibrational spectroscopy? many thanks~~!

[Jorriss]

It doesn't undergo simple harmonic motion.

[roonie1988]

It doesn't undergo simple harmonic motion.

Could you please describe it more specifically? thanks~

[Jorriss]

I can't give you specifics in the context of vibrational spectroscopy, but it might mean the periodic motion is dampened.

Take a look at this from wikipedia,
'To a first approximation, the motion in a normal vibration can be described as a kind of simple harmonic motion. In this approximation, the vibrational energy is a quadratic function (parabola) with respect to the atomic displacements and the first overtone has twice the frequency of the fundamental. In reality, vibrations are anharmonic and the first overtone has a frequency that is slightly lower than twice that of the fundamental. Excitation of the higher overtones involves progressively less and less additional energy and eventually leads to dissociation of the molecule, as the potential energy of the molecule is more like a Morse potential.'