TRILOBITE MOLECULES. New research predicts the existence of a giant two-atom molecule with an electron cloud resembling a trilobite, the ancient, hard-shelled creature which lived in the Earth's seas over 300 million years ago (see figure at Physics News Graphics). Made of two rubidium atoms spaced very far apart, the trilobite molecule could conceivably materialize in a Bose-Einstein condensate (BEC). This is because a BEC's ultracold, dense environment favors the creation of exotic species in addition to the condensate itself.

The trilobite molecule has many remarkable properties in addition to its shape, according to the collaboration that predicts its existence (Chris Greene, University of Colorado and JILA, 303-492-4770, chris.greene@colorado.edu). For starters, it would be huge for something consisting of just two atoms: the cores of the Rb atoms are separated by anywhere between 50 nm and 5 microns. Rubidium molecules in BECs have been formed before (Update 471), but they have been much smaller (only 2-4 nm).

The researchers believe the trilobite molecule can be created by manipulating a rubidium BEC with laser pulses or external electromagnetic fields. One of the rubidium atoms in the pair must first be converted into a Rydberg atom, which contains an electron in a very high orbit. Ultra-long-range molecules would then form from a weak attraction between the Rydberg atom's outermost electron and another Rb atom.

Some of these molecules would have no permanent separation of electric charge, but ones with the trilobite-shaped electron cloud could possess a large permanent electric dipole moment. With dipole moments roughly 1,000 times larger than typical polar diatomic molecules, these would be the first-ever polar molecules made up of two atoms of the same element and isotope. While extremely fragile, their large dipole moments suggest that trilobite molecules could be accelerated, transported, cooled and decelerated using much smaller electric fields than those required for any other molecule. (Greene, Dickinson, and Sadeghpour, Physical Review Letters, 18 Sept /pnu/2000/; Select Article.)