Rydberg molecule

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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.

Contents

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.

Dihelium (He2*) was the first known Rydberg molecule. [1]

Other types

In 2009, a different kind of Rydberg molecule was finally created by researchers from the University of Stuttgart. 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. [2] [3]

In 2015, a 'trilobite' Rydberg molecule was observed by researchers from the University of Oklahoma. [4] This molecule was theorized in 2000 and is characterized by an electron density distribution that resembles the shape of a trilobite when plotted in cylindrical coordinates. [5] These molecules have lifetimes of tens of microseconds and electric dipole moments of up to 2000 Debye.

In 2016, a butterfly Rydberg molecule was observed by a collaboration involving researchers from the Kaiserslautern University of Technology and Purdue University. [6] [7] A butterfly Rydberg molecule is a weak pairing of a Rydberg atom and a ground state atom that is enhanced by the presence of a shape resonance in the scattering between the Rydberg electron and the ground state atom. This new kind of atomic bond was theorized in 2002 and is characterized by an electron density distribution that resembles the shape of a butterfly. [8] As a consequence of the unconventional binding mechanism, butterfly Rydberg molecules show peculiar properties such as multiple vibrational ground states at different bond lengths and giant dipole moments in excess of 500 debye.

See also

Related Research Articles

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<span class="mw-page-title-main">Chemical bond</span> Lasting attraction between atoms that enables the formation of chemical compounds

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<span class="mw-page-title-main">Molecule</span> Electrically neutral group of two or more atoms

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<span class="mw-page-title-main">Energy level</span> Different states of quantum systems

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<span class="mw-page-title-main">Rydberg atom</span> Excited atomic quantum state with high principal quantum number (n)

A Rydberg atom is an excited atom with one or more electrons that have a very high principal quantum number, n. The higher the value of n, the farther the electron is from the nucleus, on average. Rydberg atoms have a number of peculiar properties including an exaggerated response to electric and magnetic fields, long decay periods and electron wavefunctions that approximate, under some conditions, classical orbits of electrons about the nuclei. The core electrons shield the outer electron from the electric field of the nucleus such that, from a distance, the electric potential looks identical to that experienced by the electron in a hydrogen atom.

Rydberg ionization spectroscopy is a spectroscopy technique in which multiple photons are absorbed by an atom causing the removal of an electron to form an ion.

The Rydberg states of an atom or molecule 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 that have electronic structure roughly similar to atomic hydrogen. In general, at sufficiently high principal quantum numbers, an excited electron-ionic core system will have the general character of a hydrogenic system and the energy levels will follow the Rydberg formula. Rydberg states have energies converging on the energy of the ion. The ionization energy threshold is the energy required to completely liberate an electron from the ionic core of an atom or molecule. In practice, a Rydberg wave packet is created by a laser pulse on a hydrogenic atom and thus populates a superposition of Rydberg states. Modern investigations using pump-probe experiments show molecular pathways – e.g. dissociation of (NO)2 – via these special states.

<span class="mw-page-title-main">Resonance-enhanced multiphoton ionization</span> Spectroscopy technique

Resonance-enhanced multiphoton ionization (REMPI) 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 multiphoton photoabsorption are different from the selection rules for a single photon transition. The REMPI technique typically involves a resonant single or multiple photon absorption to an electronically excited intermediate state followed by another photon which ionizes the atom or molecule. The light intensity to achieve a typical multiphoton transition is generally significantly larger than the light intensity to achieve a single photon photoabsorption. Because of this, a subsequent photoabsorption is often very likely. An ion and a free electron will result if the photons have imparted enough energy to exceed the ionization threshold energy of the system. In many cases, REMPI provides spectroscopic information that can be unavailable to single photon spectroscopic methods, for example rotational structure in molecules is easily seen with this technique.

A heavy Rydberg system consists of a weakly bound positive and negative ion orbiting their common centre of mass. Such systems share many properties with the conventional Rydberg atom and consequently are sometimes referred to as heavy Rydberg atoms. While such a system is a type of ionically bound molecule, it should not be confused with a molecular Rydberg state, which is simply a molecule with one or more highly excited electrons.

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3
. In the early universe this ability to emit infrared light allowed the primordial hydrogen and helium gas to cool down so as to form stars.

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<span class="mw-page-title-main">Rydberg polaron</span>

A Rydberg polaron is an exotic state of matter, created at low temperatures, in which a very large atom contains other ordinary atoms in the space between the nucleus and the electrons. For the formation of this atom, scientists had to combine two fields of atomic physics: Bose–Einstein condensates and Rydberg atoms. Rydberg atoms are formed by exciting a single atom into a high-energy state, in which the electron is very far from the nucleus. Bose–Einstein condensates are a state of matter that is produced at temperatures close to absolute zero.

References

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  2. Gill, Victoria (23 April 2009). "World first for strange molecule". BBC News . Retrieved 2009-04-23.
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  4. Booth, Donald; Rittenhouse, Seth; Yang, Jin; Sadeghpour, Hossein; Shaffer, James (2015). "Production of trilobite Rydberg molecule dimers with kilo-Debye permanent electric dipole moments". Science . 348 (6230): 99–102. arXiv: 1411.5291 . Bibcode:2015Sci...348...99B. doi:10.1126/science.1260722. PMID   25838380. S2CID   11508350.
  5. Greene, Chris; Dickinson, A.; Sadeghpour, Hossein (2000). "Creation of Polar and Nonpolar Ultra-Long-Range Rydberg Molecules". Physical Review Letters . 85 (12): 2458–2461. Bibcode:2000PhRvL..85.2458G. doi:10.1103/PhysRevLett.85.2458. PMID   10978081.
  6. Niederprüm, Thomas; Thomas, Oliver; Eichert, Tanita; Lippe, Carsten; Pérez-Ríos, Jesús; Greene, Chris; Ott, Herwig (2016). "Observation of pendular butterfly Rydberg molecules". Nature Communications . 7: 12820. arXiv: 1602.08400 . Bibcode:2016NatCo...712820N. doi:10.1038/ncomms12820. PMC   5059458 . PMID   27703143.
  7. Niederprüm, Thomas (2016). Rydberg-ground state interaction in ultracold quantum gases (Ph.D.). Kaiserslautern University of Technology.
  8. "Weak atomic bond, theorized 14 years ago, observed for first time".

Further reading