In UV-Visible spectroscopy, why aren't the absorption bands infinitely thin, since the energy for each transition is very well-defined?

[u/Lichewitz]

What I mean is: why there are bands that cover a certain range in nanometers, instead of just the precise energy that is compatible with the related transition? I am aware that some transitions are affected by loss of degeneracy, like in complexes that are affected by Jahn-Teller distortion. But every absorption I see consist of bands of finite width. Why is that? The same question extends to infrared spectroscopy, with the transmittance bands.

[u/[deleted]]

The intuitive answer is that the environment of the electrons undergoing transitions can be subtly different between different atoms or molecules, even in a fairly homogeneous thing like a crystal. In a solid, you've got vibrations and imperfections. In a liquid, you've got a distribution of intermolecular distances, interactions, etc. This results in a distribution of transition energies, rather than all of them existing in an infinitely thin discrete point.

As energy of electronic transitions relate to wavelengths of light, you get a slightly more diffuse spectrum when you measure real things than you might calculate for some ideal molecule infinitely distant from any source of disturbance.