New type of molecular bonding

The chemists' familiar lineup of bond types has a new member. Joining ionic, covalent, hydrogen, and van der Waals is an unusual ultralong bond between two similar atoms. The new dimers consist of rubidium atoms in their ground state and other rubidium atoms in highly excited Rydberg states. Tilman Pfau of the University of Stuttgart in Germany and his coworkers made the dimers after first trapping Rb atoms under conditions just shy of what’s needed to make a Bose-Einstein condensate. Holding the dimers together is a weak, wavy potential whose ability to bind atoms under or near BEC conditions was predicted in 2000 by Chris Greene, Alan Dickinson, and Hossein Sadeghpour. Enrico Fermi had identified the potential’s underlying interaction back in 1934, well before BECs made their laboratory debut. Because the potential is so wide and weak, it binds the two atoms only if they’re far apart. The dimers are big indeed. At 80 nm, they’re wider than a ribosome, the 5-megadalton macromolecule that translates our DNA into protein, and longer than the transistor gates in the newest, most powerful microprocessors. Pfau’s group made the simplest, most stable of the dimers that Green and company predicted. Other, more exotic dimers from the same family, dubbed trilobites (see image) and butterflies by Greene, are next on Pfau’s to-do list. (V. Bendkowksy et al., Nature , in press; http://arxiv.org/abs/0809.2961.) — Charles Day