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| 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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| The peculiar properties of the Rydberg atom come from the large charge separation and the resulting [[Hydrogen-like atom|hydrogenic]] potential. The extremely large separation between the two components of a heavy Rydberg system results in an almost perfect ''1/r'' hydrogenic potential seen by each ion. The positive ion can be viewed as analogous to the nucleus of a hydrogen atom, with the negative ion playing the role of the electron.<ref>''Heavy Rydberg states'', E. Reinhold, W. Ubachs, Molecular Physics, Vol. 103, No. 10, 20 May 2005, 1329–1352</ref>
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| == Species of heavy Rydberg system ==
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| The most commonly studied system to date is the <math>H^+/H^-</math> system, consisting of a proton bound with a <math>H^-</math> ion. The <math>H^+/H^-</math> system was first observed in 2000 by a group at the [[University of Waterloo]] in [[Canada]].
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| The formation of the <math>H^-</math> ion can be understood classically; as the single electron in a [[hydrogen atom]] cannot fully shield the positively charged nucleus, another electron brought into close proximity will feel an attractive force. While this classical description is nice for getting a feel for the interactions involved, it is an oversimplification; many other atoms have a greater [[electron affinity]] than hydrogen. In general the process of forming a negative ion is driven by the filling of [[Electron configuration|atomic electron shells]] to form a lower energy configuration.
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| Only a small number of molecules have been used to produce heavy Rydberg systems although in principle any atom with a positive electron affinity can bind with a positive ion. Species used include <math>O_2</math>, <math>H_2S</math> and <math>HF</math>. Fluorine and oxygen are particularly favoured due to their high electron affinity, high [[Ionization potential|ionisation energy]] and consequently high [[electronegativity]].
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| == Production of heavy Rydberg systems ==
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| The difficulty in the production of heavy Rydberg systems arises in finding an energetic pathway by which a molecule can be excited with just the right energy to form an ion pair, without sufficient internal energy to cause autodissociation (a process analogous to [[autoionization]] in atoms) or rapid dissociation due to collisions or local [[Electromagnetic field|fields]].
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| Currently production of heavy Rydberg systems relies on complex vacuum ultra-violet (so called because it is strongly absorbed in air and requires the entire system to be enclosed within a vacuum chamber) or multi-photon transitions (relying on absorption of multiple photons almost simultaneously), both of which are rather inefficient and result in systems with high internal energy.
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| == What makes heavy Rydberg systems interesting? ==
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| The [[bond length]] in a heavy Rydberg system is 10,000 times larger than in a typical [[Diatomic|diatomic molecule]]. As well as producing the characteristic hydrogen-like behaviour, this also makes them extremely sensitive to perturbation by external [[Electric field|electric]] and [[Magnetic field|magnetic]] fields.
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| Heavy Rydberg systems have a relatively large [[reduced mass]], given by:
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| ::<math> \mu = {m_1m_2 \over m_1+m_2} </math>
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| This leads to a very slow time evolution, which makes them easy to manipulate both spatially and energetically, while their low [[binding energy]] makes them relatively simple to detect through field dissociation and detection of the resulting [[ion]]s, in a process known as ''threshold ion-pair production spectroscopy''[1].
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| [[Kepler's laws of planetary motion|Kepler's third law]] states that the period of an orbit is proportional to the cube of the [[semi-major axis]]; this can be applied to the [[Coulomb's law|Coulomb force]]:
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| ::<math> \tau^2 = {4\pi^2\mu \over kZe^2}a^3 </math>
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| where <math>\tau</math> is the time-period, <math>\mu</math> is the reduced mass, <math>a</math> is the semi-major axis and <math>k = 1/(4\pi\epsilon_0)</math>.
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| Classically we can say that a system with a large reduced mass has a long orbital period. Quantum mechanically, a large reduced mass in a system leads to narrow spacing of the [[energy level]]s and the rate of time-evolution of the [[wavefunction]] depends on this energy spacing. This slow time-evolution makes heavy Rydberg systems ideal for experimentally probing the dynamics of quantum systems.
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| == References ==
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| <references/>
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| [[Category:Atomic, molecular, and optical physics]]
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