Atomic electronic transition
As an electronic transition, the change in the energy levels of an electron in an atom, molecule, or ( crystalline ) is called solid. If a photon emitted in this transition ( de-excitation ) or absorbed (excitation ) as referred to the transition as optical or radiative transition. When a photon is absorbed so the system goes to an excited state. Such transitions are called electronically as an electron ( in semiconductors can also be a hole meant ) his energy level changes. The Abregungsübergänge can be divided into spontaneous emission and stimulated emission.
In addition to the radiative transitions, there are non- radiative transitions, such as the Auger effect or Stoßionisationen.
Other quantum transitions are vibronic transitions where a molecule upon interaction with a photon changes its oscillation frequency and phonon -photon interaction in semiconductors.
Probability of a transition
In the quantum mechanical description of an atom, molecule or crystalline solid is possible discrete states with different energy ( energy levels ). The lowest energy state is the ground state. These states are occupied in thermodynamic equilibrium in atoms or molecules according to the Boltzmann distribution in solids, such as semiconductors, according to the Fermi distribution of electrons. In general, lower energy states are more likely to be occupied as states of higher energy. This occupation probabilities affect the transition probabilities of the transitions. In a first approximation, the transition probabilities are described by Fermi's Golden Rule. For the simplest model of a system with energy levels, the two- level system give the Einstein coefficient of the transition probabilities.
An electron can not pass by each energy level in each other. Due to the Pauli principle, two electrons may not be in the same condition, also the total angular momentum (eg, to set the angular momentum of an atom from the nuclear spin, orbital angular momentum and spin of the electrons together ) is a conserved quantity, this leads to selection rules or forbidden transitions.
Observation of electronic transitions
Experimental electronic transitions are observed for example in fluorescence spectroscopy or Raman spectroscopy. The emitted light of an excited sample is spectroscopically. The difference in energy of the electron before and after a radiative transition is carried away by a photon that contributes to a resonance line in the fluorescence spectrum. The resonance lines and the underlying electronic transitions are classified by term schemes and presented in Grotrian diagrams. The electronic transition of the hydrogen atom and the hydrogen ions are similar best understood, because these are the only atoms whose energy level can be calculated by quantum mechanics without approximation.
History
Discovered in 1802, the chemist William Hyde Wollaston dark lines in the solar spectrum, which were rediscovered in 1814 by Joseph von Fraunhofer and are called Fraunhofer lines. Gustav Robert Kirchhoff and Robert Wilhelm Bunsen introduced in 1861 found that each chemical element emits characteristic lines and declared the Fraunhofer lines as absorption lines of the elements in the upper layers of the sun. 1885 Johann Jakob Balmer was able to capture a portion of the spectral lines in the hydrogen, which are called the Balmer series with an empirical formula for the first time. This formula was generalized in 1888 by Johannes Rydberg to Rydberg formula. Electronic transitions can also be induced through electron collisions, so that in 1913 James Franck and Gustav Hertz declared with their Franck -Hertz experiment that atoms have discrete energy levels. Niels Bohr was able in 1913 to design the Bohr model of the atom, which for the first time allowed discrete energy levels of the electron and some observations on hydrogen explained. Arnold Sommerfeld extended the Bohr Model 1915/16 and was able to explain the fine structure splitting. At the same time Albert Einstein developed the Einstein rate image, which allowed a two-level system for the first time the calculation of transition probabilities. Lise Meitner discovered in 1922 radiationless electronic transitions in the context of X-ray experiments, which also independently discovered by Pierre Auger in 1926 and are now referred to as the Auger effect. In today's semiconductor devices such as photodiodes and laser diode, the specific selection of certain optical transitions plays an important role, mainly resonators are used.