An excited state is when a system is at a higher energy level than its ground state. Electrons in atoms can be excited by absorbing energy, such as a photon, and emit energy to return to the ground state. Excited states can change an atom’s chemical properties and are used in light-producing devices.
In physics, a system is said to be in an excited state if it is at an energy level higher than its base energy level, or ground state. The “system” can be an atom, molecule, ion, or other particle. When the system absorbs energy, it goes into an excited state, and when it emits energy, it returns to the ground state. The electrons in an atom, for example, exist in their ground state until they absorb energy that causes them to jump to a higher energy orbital. When this occurs, the electron would be said to be in an excited state.
Electrons, as negatively charged particles, are held by the positively charged protons in the nucleus of an atom through electromagnetic force. They surround the nucleus in a certain 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 contain a certain number of electrons. Lower energy levels tend to be filled first. When a given shell is filled, a higher energy state will begin to populate.
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 can come in the form of a photon, the basic unit of light and other electromagnetic radiation. When a photon hits the atom, the energy pushes the electron to 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 near the nucleus and decreases with distance. Electrons at the edge of the electric field, away from the nucleus, can be excited to the point of breaking free from 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 short-lived. After moving to a higher energy level, an electron usually emits a photon or phonon, a unit of light or heat, to return to its ground state. This can happen naturally, by spontaneous emission, or artificially, by stimulated emission. In rare cases, the excited state persists longer in an atom, changing its chemical properties.
Many light-producing devices are designed to excite electrons to generate photons through spontaneous or stimulated emission. Lasers, for example, work through stimulated emission. Fluorescent tubes and cathode ray tubes use spontaneous emission to produce light.
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