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, firstname.lastname@example.org or Salvador Montero,