After excitation by electromagnetic radiation, valence electrons take part in electronic transitions. Only four different transitions are allowed between σ, π and lone pair electrons.

Transition σ → σ^*

• occurs only for simple bonds (C-H or C-C for example)
• requires a high quantity of energy, and is thus found in the UV region (

Transition n → σ^*

• occurs for atoms with lone pair electrons (N, O, S….)
• requires a high quantity of energy, and is thus found between 150 and 250 nm

Transition n → π^* and n → π

• occurs only for atoms with lone pair electrons and for unsaturated functions
• there is sometimes a transition n → π when the level of the doublet is below the level of the π orbital
• these transitions usually take place in the UV domain

Transition π → π^* (aromatics)

• occurs only for atoms with unsaturated functions
• are located in the UV and visible domain (200 to 700 nm)