Observing Superfluidity in Hydrogen Molecules

Observing superfluidity in hydrogen molecules is difficult since the predicted temperature at which liquid H₂ would become superfluid (losing all viscosity), about 2 K, is well below the triple point of hydrogen (14 K), the temperature below which H₂ exists only as a solid. To make H₂ into a superfluid, H₂ molecules would have to be supercooled, cooled rapidly below their freezing point.

A new experiment at the Instituto de Estructura de la Materia-CSIC in Madrid has not yet observed superfluid H₂, but physicists there have, for the first time, proved that tiny H₂ droplets---tiny clusters, with up to 8 molecules, in a gas jet---are liquid in form.

The scientists (from Madrid, a Max Planck Institute in Goettingen, and Washington State University) determined the liquid status of the individual cluster sizes through Raman scattering, the process in which the energy of a laser beam is depleted ever so slightly when it passes through a molecular medium (in this case the H₂ droplets) by the excitation of the molecules. This proved for the first time that a Raman spectrum can be obtained for H₂ clusters.

Why so much fuss over whether hydrogen can be made superfluid? If successful it would be the first direct evidence for the existence of another superfluid besides helium, at present the only known liquid superfluid. H₂ is the simplest and most abundant molecule in the universe, and scientists rely on it to point to properties in other atoms and molecules.

Furthermore, hydrogen is the primary fuel in stars, while on Earth hydrogen might also play an important role as fuel since it has the highest chemical energy density by mass. (Tejeda et al., Physical Review Letters, 4 June 2004; contact J. Peter Toennies, 49-551-5176-600, jtoenni@gwdg.de or Salvador Montero, 39-91-590-1621, emsalvador@iem.cfmac.csic.es).