The existence of a ubiquitous class of ultra-long range molecular Rydberg states has been predicted by a JILA/ITAMP collaboration [Greene, Dickinson, and Sadeghpour, Physical Review Letters 85, 2458(2000)]. The molecular Born-Oppenheimer potential curves, rubidium dimer was chosen for demonstration, oscillate with the interatomic distance, reflecting the motion of the perturbing ground state Rb atom in the field of the Rb Rydberg electron. Two classes of ultra-long range molecules were identified: molecules of the first class are formed from the interaction between a Rb Rydberg atom with low angular momentum ( l < 3) and a distant Rb ground state atom. The equilibrium distance for these molecular states is about 1500 a.u. (~ 750 angstrom) for principal quantum number  n=30. The vibrational binding energy is about 100 MHz.

The second class of ultra-long range molecules are formed from the interaction between degenerate angular momentum states (l > 2) of the excited Rydberg atom and Rb(5s) and are much deeper in binding ( ~ 10 GHz). These molecules who equilibrium distance scales as n (for Rb(5s)-Rb(30l), the equilibrium distance is about 1200 a.u. or roughly 60 nanometer), have large permanent electric dipole moments D(R) ~ R - 0.5n² (for n=30, D_eq= 782 a. u. or about 2000 Debyes).  These giant molecules have an electron cloud resembling a trilobite, the ancient, hard-shelled creature which lived in the Earth's seas over 300 million years ago. These molecules, on the other hand, are estimated to live about 1- 10 milliseconds!

Modern-day Bose-Einstein condensates or dense magneto-optical clouds should provide favorable breeding grounds for observing these molecules. It is believed that this is a first instance in which a homonuclear molecule is predicted to exhibit a permanent electric dipole moment. Such a large dipole moment provides intriguing opportunities for  control and manipulation of these ultra-long range, ultracold molecules.
 

Media coverage:

- Neue Zurcher Zeitung  , Oct. 4, 2000 (German/Swiss Newspaper)
- "The Next Big Thing", Nature, Vol. 407, 575(2000)
- Science News, vol. 158, no. 16, 255(2000) (see also this)
- Physical Review Focus, American Physical Society
- Daily University Science News
- CERN Physics Watch
- Physics Central
 

Actual Papers:

Trilobite Molecules: Phys. Rev. Lett 2000
Butterfly Molecules: J. Phys. B Lett 2002

A trilobite picture

A butterfly picture