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What can happen to a molecule after it enters an excited state?
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photochemical reaction
Chain of fluorescent tunicates.
After a molecule absorbs energy and enters an excited state, it possesses different chemical and physical properties compared to its ground state. To return to a lower energy level, the molecule may undergo several processes:
- Radiative Transitions: The molecule can release absorbed energy by emitting a photon through fluorescence or phosphorescence.1
- Nonradiative Processes: The molecule may dissipate excess energy as heat through internal conversion (transferring electronic energy into vibrational energy) or intersystem crossing (changing electron spin states).1
- Chemical Reactions: The excited molecule may participate in a chemical reaction. It can fall apart (photodissociation), change its structure (isomerization), combine with other molecules, or transfer electrons, atoms, or excitation energy to other species.123
Because excited states often feature different bond lengths and molecular geometries, they are frequently stronger acids and stronger reductants than the original ground-state molecules, enabling critical natural processes like photosynthesis and vision.2