Giant Helium Molecules

Giant helium molecules, containing only two atoms but assuming a size as large as a small virus, have been created by researchers at the École Normale Supérieure in Paris. At sizes ranging from 10 to 100 nanometers, these helium molecules are the largest diatomic (two-atom) molecules ever created by a factor of 5 (and comparable to the size of viruses, which vary in length from 5-300 nm). What's more, helium is an inert gas that does not normally form molecules. To observe the new giant molecular states, one needs to start from an ultracold gas of atoms. At the École Normale Supérieure, researchers trap a cloud of helium atoms with magnetic fields. Each helium atom is in a long-lived "metastable" state and carries nearly 20 eV of internal energy, which is more than 10 billion times its average energy of motion. In the confines of a magnetic trap, the hottest He atoms evaporate and the colder atoms remain, lowering the temperature of the cloud to 10 microkelvins (millionths of a degree above absolute zero). Then, a laser pairs up He atoms through a process called "photoassociation," in which light of a precise color changes the state of the atoms so that they attract each other more strongly. This attraction comes about through light-induced "dipoles" (momentary separations of positive and negative charge in each He) to cause the atoms to bind to each other. To detect the molecules, the researchers record a temperature rise in the cloud that results from the successful absorption of the laser light. In a typical experiment, one percent of the atoms absorbs the light, corresponding to the formation of about 100,000 molecules. In each of the molecules, the atoms are sufficiently far apart that they resist destructive "auto-ionization" effects, in which an electron jumps from one atom to the other and breaks apart the molecule. In fact, the atoms are so distant from each other that the researchers had to account for the finite speed of light: each atom of the pair sees the other the way it was a femtosecond earlier. The researchers had to include this "retardation" effect in their calculations to get agreement with the measured data. The molecules last for an average of 50 nanoseconds--a remarkably long time due to the huge amounts of internal energy in each He atom. In precisely measuring the forces that bind the molecule, the researchers can obtain detailed information about the helium atom. In addition, the metastable helium molecule can sensitively test the accuracy of calculations in quantum chemistry, the application of quantum mechanics to chemical systems such as molecules. (Léonard et al., Physical Review Letters,15 August 2003; contact Allard Mosk, or Jérémie Léonard).