Excited state

An excited state of a physical system, each state whose energy is higher than the lowest possible, i.e., greater than the energy of the ground state. The term is mainly used in systems that only certain states can take with discrete energies, as described by quantum mechanics.

Stability and lifetime

A self on approved physical system strives experience, usually by releasing energy to the lowest energy state content to. This can be explained by the probabilities of the various system states in the phase space: A once given some form of energy ( due to friction, nuclear fission, production and emission of new particles such as photons, β - radiation etc ) returns generally only with negligible probability to the starting system back. An excited state is therefore generally not stable, but has a finite mean lifetime, in other words, a decay probability per unit time for the transition to a less highly excited state or the ground state. The mean lifetimes can be fractions of a second to thousands of years. Measured values ​​cover a range of 55 orders of magnitude, probably the largest, which occurs at a physical measure at all.

Especially long-lived excited states are sometimes referred to as metastable; see, for example isomer ( nuclear physics ).

Atomic physics

In an atom, the ground state is determined by the lowest-energy electron configuration.

By supplying energy, eg by absorption of a photon with appropriate energy ( light) or inelastic collision (Franck -Hertz experiment ), an electron can be excited to a higher energy level ( Promovierung, excitation ).

The decay into an energetically lower state ( de-excitation ) occurs either spontaneously or is triggered by an external disturbance. The released energy is given here in any form to the environment, such as:

• As a photon ( spontaneous or stimulated emission )
• Or radiationless, for example by issuing another, less tightly bound electron ( Auger electron).

In the hydrogen atom, the spins of the electron and proton can be parallel or antiparallel. The anti-parallel state having slightly higher energy is radiated at a folding back photon. This radiation is an important method of detection of astronomy for highly diluted hydrogen gas.

Examples

By collisional excitation among others, the flame color is explained by alkali and alkaline earth metals. There, the energy supply is by collisions between the atoms ( and molecules ) in the hot flame - generated - ie by heat.

Even when in gas discharge tubes (eg neon ) light produced impact excitation is in the game. The discharge current causes collisions between free electrons and the atoms.

Ionizing radiation can knock out atoms from their lattice sites. When this no longer return to its original position, arise crystal defects, which remain under circumstances long. This represents an energy storage dar. by thermoluminescence these metastable excited states can be converted back into light. Create neutron these lattice defects in graphite, it is called Wigner energy.

In order to characterize the occupation of excited states in many-body systems, is often resorted to a description by quasiparticles. For example, the excitation of lattice vibrations in a crystal is conveniently described by the generation of phonons.