Excitedstate

An excited state is a state of higher energy than a system's ground state, most often referring to electronic states of atoms and molecules. In addition to electronic excitation, vibrational and rotational excitations also exist, but the term excited state in chemistry usually denotes electronic excitation that changes the electron configuration.

Electronic excitation occurs when a particle absorbs a photon or gains energy from another source, promoting one or more electrons to higher-energy orbitals. The energy gaps define the wavelengths of light that can be absorbed and determine the ordering of excited states. States are labeled by spin multiplicity and symmetry terms; common categories include singlet and triplet electronic states.

Relaxation from an excited state can proceed radiatively, emitting a photon (fluorescence from singlet states; phosphorescence from triplet states), or non-radiatively through internal conversion or intersystem crossing, converting energy into heat. Lifetimes span from picoseconds to seconds depending on the mechanism and environment.

Experimental probes include UV–visible absorption spectroscopy, which detects transitions from the ground state to excited states, and fluorescence or phosphorescence spectroscopy, which monitors emission. Time-resolved techniques reveal lifetimes and relaxation pathways.

Theoretical descriptions use quantum chemistry methods. Time-dependent density functional theory (TD-DFT) is widely used to predict excited-state energies and spectra; more accurate multireference methods are employed for challenging cases. Potential energy surfaces of excited states govern photochemical reactions and energy transfer processes.

Excited states play central roles in many phenomena and technologies, including vision and photosynthesis, fluorescent imaging, light-emitting diodes, solar cells, and photocatalysis. Understanding their properties enables control over light-supported chemistry and materials design.