Editing Ultraviolet–visible spectroscopy (section)

==Optical transitions==
Most molecules and ions absorb energy in the ultraviolet or visible range, i.e., they are [[chromophore]]s.  The absorbed photon excites an electron in the chromophore to higher energy molecular orbitals, giving rise to an [[excited state]].<ref name= pri>{{cite web |last= Metha |first= Akul |title= Principle |website= PharmaXChange.info |date= 13 Dec 2011 |url= http://pharmaxchange.info/press/2011/12/ultraviolet-visible-uv-vis-spectroscopy-principle/}}</ref> For organic chromophores, four possible types of transitions are assumed: π–π*, n–π*, σ–σ*, and n–σ*.  Transition metal complexes are often colored (i.e., absorb visible light) owing to the presence of multiple electronic states associated with incompletely filled d orbitals.<ref name=RSD>{{cite book |title=Physical Methods for Chemists, 2nd Edition|author=R. S. Drago|isbn=0030751764|publisher=W. B. Saunders|year=1992}}</ref>