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Astron. Astrophys. 358, 276-286 (2000)

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3. Deexcitation of Rydberg Matter

The electrons which are excited into the partly delocalized orbitals in RM also give the bonding between the atoms in RM, as in an ordinary metal. The various energy levels for the core level electrons close to the core ions, the RM electrons in the conduction band between the atoms, and the empty intermediate electronic states around the core ions are shown in Fig. 2. The conduction band is approximately half-filled, and the energy required to move an electron from the highest occupied orbital in the conduction band (at the Fermi level) to infinite distance is the surface work function. This energy is small, of the order of a few K at excitation levels of n = 80.

[FIGURE] Fig. 2. Emission of light from RM by the two-electron mechanism, Eq. (2), in which one electron falls down from the RM Fermi level to an intermediate Rydberg state at the same time as another electron is excited from one Rydberg state to another, higher one.

A direct deexcitation process

[EQUATION]

with emission of a photon corresponding to the energy difference between a level in the RM continuum and a rather low Rydberg state in the atom or molecule M is not very likely due to the very small overlap between these states. However, if electrons are released in the RM by for example energetic particles, or a free electron current passes through the RM due to external accelerating fields, a direct capture of this type may take place. The final state is indicated as nR, which means a Rydberg electron with principal quantum number n and maximum angular momentum quantum number l. There is no selection rule for the n value, but a selection rule for the angular momentum quantum number l must exist, with the change [FORMULA] = [FORMULA]. This process is a deexcitation process for RM, since the final result is a Rydberg atom or molecule separated from the RM. It is the simplest process giving transitions of relevance to the UIR bands, but a slightly more complex process is also likely, as shown below.

Two-electron processes are observed in laser Raman spectroscopic experiments in RM (Svensson & Holmlid 1999). This means that when one electron falls down from the RM states, a Rydberg electron in the same atom is excited slightly with a simultaneous large change in the value of l. The complete process may then be written

[EQUATION]

where [FORMULA] +[FORMULA] - [FORMULA] - [FORMULA] = [FORMULA] should be valid. This process is depicted in Fig. 2. The main contribution to the emitted light would come from the energy change from the RM level at [FORMULA] to a low value [FORMULA] of one electron. The energy change from a high value [FORMULA] to another relatively high value [FORMULA] may be rather small, since the Rydberg energy levels vary as [FORMULA] relative to the ionization limit. This kind of process also implies a deexcitation of the RM, since the final state is a neutral atom or molecule, albeit in a doubly excited state. Of course, this type of process can only exist if the central core in the M atom or molecule has at least two charges, which excludes H atoms. Hydrogen molecules on the other hand may take part in such a process, but may suffer dissociation after the electron transfer is completed.

The RM surviving for a long time at high n values in the range n = 50 - 90 and large binding distances, 0.4-1.2 µm, may finally deexcite by sequential emission of many IR quanta to lower RM states to which the overlap has increased above some minimum value. This means, that the IR transitions to the low n values may take place from upper states with a lower state of excitation than for radiative transitions to not so low n values.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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