Observable transitions in atoms and molecules

Different atoms and molecules absorb radiation of different wavelengths. An absorption spectrum will exhibit a number of sharp lines (specifically for 

Absorption of UV and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy. The spectmm of a molecule containing these chromophores is complex, due to the superposition of rotational and vibrational transitions on the electronic transitions. These result in a combination of overlapping lines, which have the 

Most absorption spectroscopy of organic compounds is based on transitions of n or tt electrons to the tt excited state. Their absorption peaks fall into the experimentally convenient wavelength region A S 

200—700 nm. Note that these transitions need an unsaturated group in the molecule to provide the tt electrons. Molar absorbtivities associated with n — tt transitions are low, normally of the order of e = 10—lOOmor cm. For tt — tt transitions one 

For a detailed discussion of all electro-transfer mechanisms in molecules you may consult standard textbooks on molecular spectroscopy, such as Hollas (2003). 

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