Ultra-Long Range Molecules

Pushing chemical bonding to an extreme, a group led by Vera Bendkowsky and Tilman Pfau of the University of Stuttgart, in Germany, has created rubidium dimers with 100-nm bond lengths (Nature 2009, 458, 1005).

Molecules with such ultralong bonds had been predicted theoretically but never before observed experimentally. Normal Rb₂ has a bond length of about 0.4 nm.

The new dimers are fleeting, with a lifetime of only about 18 microseconds at microkelvin temperatures. Nevertheless, "the experimental proof of the existence of ultra-long-range molecules is truly a remarkable achievement" that could lead to a new kind of ultracold chemistry, says Jan M. Rost of the Max Planck Institute for the Physics of Complex Systems, in Germany.

The scientists prepared the new Rb₂ molecules by using a laser pulse to excite one Rb atom into a so-called Rydberg state, in which at least one electron has a high principal quantum number—34 to 40 in these experiments. That far-flung electron can then form a bond with a distant rubidium atom in the ground state, creating a Rydberg molecule.

In a commentary about the work, Chris H. Greene of the University of Colorado, Boulder, likens the outermost electron to "a sheepdog that keeps its flock together by roaming speedily to the outermost periphery of the flock and nudging back towards the center any member that might begin to drift away."