Ultraviolet spectroscopy HOMO-LUMO transition

Electromagnetic radiation can excite electrons from the highest occupied molecular electronic orbitals (HOMO) to the lowest unoccupied orbitals (LUMO), if the fiequency of the radiation corresponds to the resonance energy of this transition. This frequency relates to a wavelength in the ultraviolet and visible. Thus, sometimes this type of excitation is called electron spectroscopy or absorption spectroscopy in the UV/Vis. The wavelength of the maximum of absorption correlates with the electronic structure of the examined molecules. 

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