Rydberg atom

A Rydberg state ( after the Swedish physicist Johannes Rydberg ) is a quantum mechanical state of an atom, ion or molecule in which the outermost electron is much further from the center than in the ground state. An atom in such a state is called a Rydberg atom and described already in the simplest quantum mechanical approach to the hydrogen problem well.

In accordance with the correspondence principle goes for large quantum numbers, the quantum mechanical description of the Rydberg atom in the classical description above. In fact, the electron can be treated here in a good approximation as a classical particle, as it is based on the Bohr model or Bohr - Sommerfeld model of the atom.

Due to their large compared to ordinary atoms expansion and large number of closely spaced or ( almost) degenerate energy levels of Rydberg atoms are particularly sensitive to electric and magnetic fields. So shows a Rydberg atom which crosses through it through a mirrored cavity with one trapped inside photon changes its wave function. Thus, for example, the presence of the photon be detected without affecting it further (so-called quantum non demolition measurement). For the development of applications based on experimental methods of otherwise unavailable sensitivity and accuracy Serge Haroche and David Wineland received the Nobel Prize for Physics in 2012.

Mark

From Rydberg state occurs when an atom or molecule is excited so that an electron has a principal quantum number, which is far above the atoms occurring in the ground state maximum value. If the electron is also a correspondingly high angular momentum ( maximum angular momentum quantum number ), its probability in the vicinity of the core and optionally the other electrons of the atom is very small so that they fit together very well as a single point charge and other subtleties of the interactions with the core and the other electrons play a very small role. Therefore Rydberg states of all atomic species correspond very closely the simple conditions for the hydrogen atom. In particular, the energy is given well and degenerate with respect to the orbital angular momentum.

The energy of an electron in a Rydbergzustand is only slightly below the level of vacuum and thus is much higher than the energy from further inward electrons have a higher binding energy. This also means that the corresponding electron very easily separated from the atom ( ionized) can be.

These high-lying energy levels can be occupied (eg with suitable radiation wavelength) by electronic excitation. But Rydberg states may also arise if an ion capturing an electron, for example, if the ion is close to a surface and passes an electron therefrom to the ion.

Rydberg state at the outermost electron of a molecule in a molecular orbital, which is composed of atomic orbitals that are not part to the valence shell of the molecule.

Proportions

When electrons in far away from the core Rydbergzuständen many properties can by classical physics, that is, without consideration of quantum physics, are described. Therefore, for the distance in a proton-electron Rydberg hydrogen atom:

With the Bohr radius.

This Rydberg atoms are very large, eg, for n = 100:

The largest quantum numbers are reached at n ≈ 500 with atomic diameters of almost one hundredth of a millimeter.

Binding energies

The more the electron is removed from the proton, the weaker it is attached and is the smaller the necessary release of energy:

With the Rydberg energy

It follows that, for N = 100 are already sufficient thermal energy in order to separate the electron permanently. For this reason, one can produce such highly excited atoms only in high vacuum and " keep ". They arise naturally in the upper atmosphere of the Earth or the stars.

Residence time

Rydberg atoms are classic examples of a population inversion, because most or all lower states are empty. Especially in the absence of collisions with other atoms and at maximum orbital angular momentum of the electron can be great durability. The electron can reduce its orbital angular momentum quantum number by emitting a photon only by 1 and therefore has to jump to the next lower shell with quantum number. However, their energy differs so little from that the emission of the corresponding long-wavelength photon is greatly hindered. Therefore, the spectral lines of Rydberg atoms were first discovered in very dilute stellar atmospheres or interstellar gas, where the atoms do not collide during a sufficiently long period of time with another atom.

Example

In the hydrogen atom the 1s shell is the valence shell. For the molecule can be derived from the 1s atomic orbitals of the two atoms, the molecular orbitals and construct. However, can be constructed such molecular orbitals, which are then referred to as Rydberg states also from the unoccupied in the atom 2s, 2p, 3s, ... atomic orbitals.