Focus: Measuring a Fragile Molecule

Phys. Rev. Focus 6, 9
A team of physicists has precisely measured the length and strength of the exceedingly delicate bond formed between two helium atoms.

Although theory has long predicted that two helium atoms can form a diatomic molecule, or dimer, experimental evidence was elusive. Traditional probes of atomic structure don’t work for helium because they tear apart the fragile molecule like a butterfly blasted with a shotgun. Now, in the 11 September PRL, researchers describe a non-destructive technique that confirms that the atoms in a helium dimer form the longest and weakest chemical bond known, and the largest two-atom molecule.

Helium is only one step in the periodic table away from hydrogen, which forms nature’s most common dimer. “Helium is a very fundamental atom,” says Harvard University’s Isaac Silvera, “so we can make very accurate calculations of its bound states.” Those calculations showed that the helium-helium attractive force should create a dimer state with a tiny binding energy of about ${10}^{-7}e\phantom{\rule{0}{0ex}}V,$ compared with 5 eV for diatomic hydrogen.

The weak bond makes it nearly impossible to examine the helium dimer. Traditional particle probes of atomic structure–microwave, infrared, and visible light spectroscopy, x-ray diffraction, and electron scattering–are too powerful; the necessary electron and photon collisions instantly smash the fragile dimer in two. So it wasn’t until 1994 that Peter Toennies and his colleagues at the Max Planck Institute in Göttingen, Germany, convinced most researchers that the dimer existed at all. They produced a diffraction pattern from an ultracold beam of helium atoms and dimers, but this experiment didn’t measure the bond length, the crucial parameter from which the binding energy is derived. In 1996 Ron Gentry and his colleagues at the University of Minnesota sifted helium through a nanoscale sieve and esitmated 62 Å for the bond length, a value some experts considered to be an upper limit.

Now a team led by Toennies and Gerhard Hegerfeldt of the University of Göttingen has filled in the last piece of the helium dimer puzzle. To measure the dimer bond length, they launched a 4.5 K beam of helium atoms towards a diffraction grating. In flight, about 5% of the atoms formed dimers as the beam cooled to less than 1 mK. On passing through the 70-nm-wide slits in the grating, the cold beam produced a series of alternating large and small diffraction peaks corresponding to helium atoms and the helium dimer, respectively. The peak intensities indicated a bond length of 52 Å, remarkably close to the classical estimate from four years ago. “It is like putting calipers on the molecule,” says Toennies. And a simple quantum mechanical computation led to the binding energy of $9.5×{10}^{-8}e\phantom{\rule{0}{0ex}}V$ “This is a beautiful, elegant, and gratifying confirmation of our work,” says Gentry. Dick Manson of Clemson University in South Carolina agrees that it confirms the previous estimate based on classical physics, while adding quantum corrections. Silvera is also impressed. “Toennies’s team has done a very nice experiment and analysis,” he says. “It completes the picture of the helium dimer.”

–Mark Sincell

Mark Sincell is a freelance science writer based in Houston, TX.

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