Energy level

An energy level of the discrete energy associated as an energy eigenstate of the quantum-mechanical state of a system (such as an atom or an atomic nucleus ). Energy levels are the eigenvalues ​​of the Hamiltonian, therefore they are independent of time. The system can be permanently only in one of these states, but not in others, intermediate values ​​of energy " held back". A self- over -ground system always seeks its lowest energy level, the ground state, too. All other levels are called excited states.

Intuitively, one can imagine that arrangement and movement, the electrons in the atomic shell - or the nucleons in the nucleus - each are stable only in a very specific form. Each of these states has a different, specific energy content. However, there are above a certain threshold energy and an energy continuum, a range of possible arbitrary energy values ​​. This limit corresponds exactly to the binding energy of the most easily separable particle ( see, eg, ionization). The continuum of possible energies arises from the fact that this separated particles can fly away with any kinetic energy. A special situation occurs in the continuum of certain energy, for which there is also an excited state in which no particle was separated. Such a condition is sometimes referred to as " bound state within the continuum ." He makes, for example, as a resonance in the cross section of shock reactions noticeable ( in nuclear reactions see compound nucleus ).

Transitions between energy levels

Energy consumption in the system can only be done by changing to a higher energy level or in the continuum. This is done for example by absorption of a photon or by inelastic collision of a particle as in the Franck -Hertz experiment. As transitions between discrete levels here, the most suitable amount of energy must be supplied; the process is called excitation. It leads to discrete absorption lines in the spectrum.

The reverse transition from a higher to a lower level with the release of a photon can be stimulated or spontaneously from the outside. The spontaneous process is called excited-state decay or spontaneous emission. Like the radioactive decay it is characterized by a half time. The energy of the emitted photon corresponds to the energy difference between the two energy levels involved. This causes the discrete spectral lines of the emission spectrum of the excited atoms and molecules. Excited states and decay with the emission of electromagnetic radiation, there are not only atoms, but also in atomic nuclei. In these, the energy differences between the states are significantly higher. With them is caused by spontaneous emission, therefore, the high-energy gamma radiation.

An emission operation, which does not occur spontaneously, the stimulated emission is used with the laser.

Due to the selection rules are not allowed all the transitions in some cases.

Energy levels in the atom

The energy levels of the atoms described by the principal quantum number n. The energy of the state with the quantum number n in a hydrogen-like atom of atomic number Z is approximately

With the Rydberg energy ER = 13.6 eV.

There are a further fine-structure and hyperfine-structure corrections and the Lamb shift.