Giant helium molecules

Have been created by researchers at the Ecole Normale Supérieure in Paris. With atomic separations ranging from 8 to 60 nanometers, the diatomic molecules are comparable to the size of small viruses. Normally, He is chemically inert. To create the new giant molecular states, the researchers first cooled a gas of He atoms to 10 µK. Each atom was in a long-lived metastable state and carried nearly 20 eV of internal energy, more than 10¹⁰ times its average energy of motion. The physicists then used a laser to pair up He atoms through photoassociation, a process in which light-induced dipoles cause two atoms to bind to each other. To detect the molecules, the researchers recorded a temperature rise in the cloud; that increase resulted from the successful absorption of the laser light. In a typical experiment, 1% of the atoms absorb the light and form about 10⁵ molecules. The atoms in each molecule are so far apart that they resist destructive auto-ionization effects in which an electron jumps from one atom to the other. To get theory to agree with the measured data, the researchers had to account for the finite speed of light through a retardation effect. The molecules lasted for a surprisingly long 50 nanoseconds. (J. Léonard et al. , Phys. Rev. Lett. 91 , 073203, 2003 .)