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UV-vis absorption spectroscopy is a technique used to study molecules which absorb ultra violet to visible light. These molecules are usually highly conjugated and thus contain many interconnecting π-bonds. The electron in one of the π-bonds can be excited to a higher antibonding orbital (which is empty). I.e. if the energy difference between the bonding and antibonding orbital is equal to that of the UV-vis light imputed into the system, the molecule is said to absorb the UV-vis light and therefore will be able to produce an absorption spectrum. Only molecules with the correct energy difference between the bonding and antibonding orbitals will therefore be able to absorb UV-vis light (where the wavelength is 200-800nm). Having conjugated systems of interconnecting π-bonds allows the optimum difference between the bonding and antibonding orbitals and is why UV-vis absorption spectroscopy is a good technique to use to study these types of molecules. See link to learn more about UV-vis absorption spectroscopy.

UV-vis absorption spectroscopy is also associated with fluorescence and luminescence. This is some ways the opposite process, where by molecules that have been excited to a higher energy state de-excite, thus emitting light of a specific frequency. This can be investigated with luminescence spectroscopy. To learn more about fluorescence and luminescence see the related chemBAM page.

Page Author; Thomas Squire

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