Why is UV-Vis Spectroscopy not like IR Spectroscopy UV/Vis Spectroscopy is where electromagnetic radiation in the UV to Visible range is shone at a sample to be analysed. The absorbance of each wavelength is recorded and then plotted (very similar to Infrared spectroscopy). That's why it surprised me so much.



This graph is so smooth and with only 1 peak. This is completely different to the IR graph (below):


Why is it that UV-Vis spectroscopy only produces 1 peak? Is it something to do with orbitals overlapping and hence only producing one major peak?
I'm collecting ideas from the excellent comments and adding some of my own:

The typical UV/Vis spectrum has a narrow range

On the low energy side, it ends where the visible spectrum ends, at about 800 nm. On the high energy side, it ends where water and quartz start absorbing too much, at about 220 nm. Compared to an IR spectrum, it has a narrow range.

Most functional groups have no electronic transitions in that range

As far as organic compounds are concerned, the most common functional group to absorb in the UV is the benzene ring, and extended conjugated systems (e.g. beta carotene) will have electronic transitions in the visible. On the other hand, most organic functional group will show vibrational transitions in the range of a typical IR spectrum.

Peak widths

The spectrum of an electronic transition is broad because it is coupled with vibrational and rotational transitions. An example is the electronic spectrum of iodine:



Source: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Map%3A_Physical_Chemistry_(McQuarrie_and_Simon)/13%3A_Molecular_Spectroscopy/13-06._Electronic_Spectra_Contain_Electronic%2C_Vibrational%2C_and_Rotational_Information

In solution, the bands are not resolved, so the spectrum appears as a single broad peak.