In physics, a system is said to be in an excited state if it is at a higher energy level than its baseline energy level, or ground state. The “system” may be an atom, molecule, ion, or other particle. When the system absorbs energy, it transitions into an excited state, and when it emits energy, it reverts to a ground state. Electrons in an atom, for example, exist at their ground state until they absorb energy that causes them to jump to a higher energy orbital. When this occurs, the electron would then be said to be in an excited state.
Electrons, as negatively charged particles, are held to the positively charged protons in the nucleus of an atom through electromagnetic force. They surround the nucleus in a number of atomic orbitals, each of which corresponds to a discrete energy level. Each orbit around the atomic nucleus, conceptualized as an electron shell, can only hold a certain number of electrons. The lowest energy levels tend to be filled first. When a given shell is filled, a higher energy state will begin to be populated.
It is possible for an electron to jump to a higher energy level before that level is populated, but this requires energy from outside the system. This energy may come in the form of a photon, the basic unit of light and other electromagnetic radiation. When a photon strikes the atom, the energy propels the electron into a higher energy level.
An electron requires more energy to jump from the first energy level to the second than from the second to the third. This is because the attractive force of the nucleus’ electric field is strongest close to the nucleus, and decreases with distance. Electrons at the very fringe of the electric field, far from the nucleus, can be excited to the point of breaking free of the atom entirely. When this happens, the atom loses that unit of negative charge and becomes ionized — in other words, it is no longer neutrally charged, but instead becomes a positively charged ion.
The excited state is often brief. After jumping to a higher energy level, an electron will usually emit a photon or phonon — a unit of light or heat — to return to its ground state. This can happen naturally, through spontaneous emission, or artificially, through stimulated emission. In rare cases, the excited state is preserved for longer in an atom, modifying its chemical properties.
Many light-producing devices are designed to excite electrons to generate photons through spontaneous or stimulated emission. Lasers, for example, function through the stimulated emission. Fluorescent tubes and cathode ray tubes use spontaneous emission to produce light.