Excitasjon

Excitasjon (excitation) is the process by which a system gains energy and moves from a lower-energy state to a higher-energy state. In atomic and molecular physics, excitation occurs when a particle absorbs energy—typically a photon or energy transferred in a collision—that promotes electrons to higher electronic, vibrational, or rotational levels. The excited state is usually unstable and relaxes back, often emitting radiation (fluorescence or phosphorescence) or transferring energy non-radiatively to the surroundings.

In spectroscopy, excitation refers to the absorption that induces transitions, producing excitation spectra that reveal energy-level spacings and transition probabilities. In chemistry and photochemistry, excited states can enable reactions or alter pathways not accessible from the ground state.

In solid-state physics, excitation can mean promotion of electrons across an energy gap, creating charge carriers. A related concept is the exciton, a bound electron–hole pair that can form in semiconductors and insulators.

In neuroscience, excitation describes the process by which a neuron becomes more likely to fire an action potential, typically via excitatory synaptic inputs that depolarize the membrane; inhibition reduces excitability.

Lifetimes of excited states vary from femtoseconds to milliseconds or longer, depending on the system and environment. Relaxation can be radiative, emitting photons, or non-radiative, dissipating energy as heat. Ionization—removing an electron entirely—contrasts with excitation.