excitations

An excitation is a state in which a system possesses more energy than its ground state. In quantum systems, excitations are quantized and correspond to the absorption of energy that promotes the system to a higher allowed energy level or to a collective mode. Excitations can be created by external perturbations such as photons, particle collisions, thermal energy, or applied fields, and they typically decay back toward the ground state over characteristic lifetimes.

Different systems support different kinds of excitations. In atoms and molecules, electronic excitations involve promoting electrons to higher electronic orbitals; vibrational excitations involve quantized vibrational motion of nuclei; rotational excitations involve rotation of molecules. In solids, electrons can be excited across energy bands, creating electron-hole pairs; quantized lattice vibrations give phonons; collective spin oscillations give magnons; bound electron-hole pairs are excitons; charge-density oscillations are plasmons.

Excitations are often described by their energy spectrum E(k) or E, their momentum, and their lifetime. Theoretical treatments use quantum mechanics and many-body techniques, including perturbation theory, Green's functions, and various lattice or band-structure models. Experimental probes include spectroscopy, photoemission (ARPES), neutron scattering, infrared and Raman techniques, and inelastic light scattering, which reveal excitation energies and dispersions.

Commonly cited examples include electronic transitions in atoms and semiconductors, phonons in crystals, and excitons in semiconductors. Excitations play a central role in many technologies, such as lasers, light-emitting diodes, solar cells, and sensors, by controlling how energy is absorbed, transported, or converted.

Lifetime and decay pathways influence device performance, with some excitations persisting as long-lived states or contributing to transport phenomena like superconductivity, metal-insulator transitions, or collective quantum phases.