Rydberg molecules
Encyclopedia
A Rydberg molecule is an electronically excited chemical species
. Electronically excited molecular states are generally quite different in character from electronically excited atomic states. However, particularly for highly electronically excited molecular systems, the ionic core interaction with an excited electron can take on the general aspects of the interaction between the proton and the electron in the hydrogen atom. The spectroscopic assignment of these states follows the Rydberg formula
, named after the Swedish
physicist Johannes Rydberg
, and they are called Rydberg states
of molecules. Rydberg series are associated with partially removing an electron from the ionic core.
Each Rydberg series of energies converges on an ionization energy threshold associated with a particular ionic core configuration. These quantized Rydberg energy levels can be associated with the quasiclassical Bohr atomic picture. The closer you get to the ionization threshold energy, the higher the principal quantum number, and the smaller the energy difference between near threshold Rydberg states. As the electron is promoted to higher energy levels in a Rydberg series, the spatial excursion of the electron from the ionic core increases and the system is more like the Bohr quasiclassical picture.
The Rydberg states of molecules with low principal quantum numbers can interact with the other excited electronic states of the molecule. This can cause shifts in energy. The assignment of molecular Rydberg states often involves following a Rydberg series from intermediate to high principal quantum numbers. The energy of Rydberg states can be refined by including a correction called the quantum defect in the Rydberg formula. The quantum defect correction can be associated with the presence of a distributed ionic core.
The experimental study of molecular Rydberg states has been conducted with traditional methods for generations. However, the development of laser-based techniques such as Resonance Ionization Spectroscopy has allowed relatively easy access to these Rydberg molecules as intermediates. This is particularly true for Resonance Enhanced Multiphoton Ionization (REMPI
) spectroscopy, since multiphoton processes involve different selection rules from single photon processes. The study of high principal quantum number Rydberg states has spawned a number of spectroscopic techniques. These "near threshold Rydberg states" can have long lifetimes, particularly for the higher orbital angular momentum states that do not interact strongly with the ionic core.
Rydberg molecules can condense to form clusters of Rydberg matter
which has an extended lifetime against de-excitation.
. There, the interaction between a Rydberg atom
and a ground state atom leads to a novel bond type
. Two rubidium
atoms were used to create the molecule which survived for 18 microseconds.
Chemical species
Chemical species are atoms, molecules, molecular fragments, ions, etc., being subjected to a chemical process or to a measurement. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on a...
. Electronically excited molecular states are generally quite different in character from electronically excited atomic states. However, particularly for highly electronically excited molecular systems, the ionic core interaction with an excited electron can take on the general aspects of the interaction between the proton and the electron in the hydrogen atom. The spectroscopic assignment of these states follows the Rydberg formula
Rydberg formula
The Rydberg formula is used in atomic physics to describe the wavelengths of spectral lines of many chemical elements. It was formulated by the Swedish physicist Johannes Rydberg, and presented on November 5, 1888.-History:...
, named after the Swedish
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
physicist Johannes Rydberg
Johannes Rydberg
Johannes Robert Rydberg, , , was a Swedish physicist mainly known for devising the Rydberg formula, in 1888, which is used to predict the wavelengths of photons emitted by changes in the energy level of an electron in a hydrogen atom.The physical constant known as the...
, and they are called Rydberg states
Rydberg states
The Rydberg states of an atom are electronically excited states with energies that follow the Rydberg formula as they converge on an ionic state with an ionization energy. Although the Rydberg formula was developed to describe atomic energy levels, it has been used to describe many other systems...
of molecules. Rydberg series are associated with partially removing an electron from the ionic core.
Each Rydberg series of energies converges on an ionization energy threshold associated with a particular ionic core configuration. These quantized Rydberg energy levels can be associated with the quasiclassical Bohr atomic picture. The closer you get to the ionization threshold energy, the higher the principal quantum number, and the smaller the energy difference between near threshold Rydberg states. As the electron is promoted to higher energy levels in a Rydberg series, the spatial excursion of the electron from the ionic core increases and the system is more like the Bohr quasiclassical picture.
The Rydberg states of molecules with low principal quantum numbers can interact with the other excited electronic states of the molecule. This can cause shifts in energy. The assignment of molecular Rydberg states often involves following a Rydberg series from intermediate to high principal quantum numbers. The energy of Rydberg states can be refined by including a correction called the quantum defect in the Rydberg formula. The quantum defect correction can be associated with the presence of a distributed ionic core.
The experimental study of molecular Rydberg states has been conducted with traditional methods for generations. However, the development of laser-based techniques such as Resonance Ionization Spectroscopy has allowed relatively easy access to these Rydberg molecules as intermediates. This is particularly true for Resonance Enhanced Multiphoton Ionization (REMPI
REMPI
Resonance Enhanced Multi-Photon Ionization is a technique applied to the spectroscopy of atoms and small molecules. In practice, a tunable laser can be used to access an excited intermediate state. The selection rules associated with a two-photon or other multi-photon photoabsorption are different...
) spectroscopy, since multiphoton processes involve different selection rules from single photon processes. The study of high principal quantum number Rydberg states has spawned a number of spectroscopic techniques. These "near threshold Rydberg states" can have long lifetimes, particularly for the higher orbital angular momentum states that do not interact strongly with the ionic core.
Rydberg molecules can condense to form clusters of Rydberg matter
Rydberg matter
Rydberg matter is a phase of matter formed by Rydberg atoms; it was predicted around 1980 by É. A. Manykin, M. I. Ozhovan and P. P. Poluéktov. It has been formed from various elements like caesium, potassium, hydrogen and nitrogen; studies have been conducted on theoretical possibilities like...
which has an extended lifetime against de-excitation.
Other types of Rydberg molecules
In 2009, a different kind of Rydberg molecule was finally created by researchers from the University of StuttgartUniversity of Stuttgart
The University of Stuttgart is a university located in Stuttgart, Germany. It was founded in 1829 and is organized in 10 faculties....
. There, the interaction between a Rydberg atom
Rydberg atom
thumb|right|300px|Figure 1: Energy levels in atomic [[lithium]] showing the Rydberg series of the lowest 3 values of [[Angular momentum#Angular momentum in quantum mechanics|orbital angular momentum]] converging on the first ionization energy....
and a ground state atom leads to a novel bond type
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
. Two rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
atoms were used to create the molecule which survived for 18 microseconds.
Further reading
- Molecular Spectra and Molecular Structure, Vol. I, II and III Gerhard Herzberg, Krieger Pub. Co, revised ed. 1991.
- Atoms and Molecules: An Introduction for Students of Physical Chemistry, Martin Karplus and Richard N. Porter, Benjamin & Company, Inc., 1970.