[CHEKAL]

If an absorption was really broad, what would it tell us able the nuclear geometry of the electronic ground and excited states to one another and how is this different to a sharp absorption band? The band is around 400 nm.

[Gibbs]

Read up the relatively simple Frank-Condon factor to explain the nuclear geometry part of your question. hint: Broad peak means dissimilar nuclear geometry. A broad absorption in a molecule (NOT ATOM) is due to the fact you have rotational and vibrational energy levels superimposed on your electronic energy levels. Each transition is slightly different from the other in energy, thus the wavelength of absorption is slightly different and you get a broadening of peak. You would expect sharp peaks in atomic spectroscopy.