excitation

Excitation is the process of supplying energy to a system to raise it from a lower-energy state to a higher-energy state. In physics and chemistry, excitation most often refers to electronic, vibrational, or rotational excitation of atoms or molecules. In neuroscience, excitation denotes processes that increase neuronal membrane potential toward firing. In electrical engineering, excitation refers to providing a driving current or field to a device such as a generator or motor to produce a response. The concept centers on moving the system away from thermodynamic equilibrium by adding energy.

In quantum systems, excitation occurs when a system absorbs energy equal to the difference between energy levels. This energy is delivered in discrete quanta, such as photons or phonons, or through collisions. The excited state is usually unstable and decays to lower energy states, releasing energy as light or heat. De-excitation can occur radiatively, producing fluorescence or phosphorescence, or nonradiatively through internal conversion or vibrational relaxation.

Types of excitation include electronic excitation, where electrons are promoted to higher orbitals; vibrational excitation, raising molecular vibrational energy; and rotational excitation, changing rotational states. Thermal excitation arises from population of higher-energy states according to temperature. In materials and devices, electrical or magnetic excitation involves applying currents or fields to create desired states, such as magnetic polarization or bright emission.

In neuroscience, excitation refers to depolarization that brings a neuron toward the threshold for an action potential, typically driven by synaptic input. In spectroscopy and imaging, excitation spectra describe how efficiently a system is excited across wavelengths, while emitted light reveals the resulting de-excitation processes. Excitation underpins technologies ranging from lighting and lasers to sensors and electric machines.