Theoretically, since the differences between electronic energy levels of two electronic states are well defined, one should obtain sharp, well-defined peaks from UV-VIS. Practically, the transition of an electron from one energy level to another is always accompanied by simultaneous change in vibrational and rotational states also and causes transitions between various vibrational and rotational levels of lower and higher energy electronic states. Therefore many radiations of closely placed frequencies are absorbed and a broad absorption band is obtained. So, band would be a more appropriate term for UV-VIS absorptions.

Thank you so much Professor Catalin Croitoru for your good sharing. Indeed, I do appreciate that.

Best regards,

Mostafa

Peak versus band

Bands

Almost all molecules have an absorption spectrum with (a few) broad bands

The distance in energy between the bands is constant

This difference is equal to the energy of a vibration.

Peak

A peak is much narrower than a band. Atoms and small molecules have a spectrum with narrow peaks

Confusion

In an absorption spectrum of a molecule the band with the highest absorption is often called the peak

For background information you should study a textbook on UV-Vis spectroscopy with examples

or Google for :vibronic transitions; Franck-Condon principle