Laser excited NO entrained in a supersonic molecular beam
evolves to form an ultracold plasma of molecules
When a tunable laser pulse promotes a large fraction of the atoms held in a magneto-
optical trap (MOT) to a high Rydberg state (n > 30), the ensemble spontaneously evolves
on a microsecond timescale to produce an ultracold plasma. This phenomenon has attracted
great interest because the plasma that forms o ers a means to study atomic ion-electron
interactions under conditions of charged-particle correlation otherwise found only in ther-
monuclear explosions and the cores of dense stars.
Molecular ion - electron systems are potentially even more interesting. They present
many more channels of interaction, and one can expect the plasma formed from a molecular
Rydberg gas to exhibit novel forms of excitation transfer and dissipation, including electron-
cation inelastic scattering and dissociative recombination. In principle, the mild conditions
of an ultracold plasma ought to readily support the development of a Coulomb-correlated
population of electrons and molecular cations. However, unlike atoms, a gas of cold molecules
cannot be prepared for laser excitation in a MOT, and until now, no one has produced an
ultracold plasma of molecules for study.
We have overcome this problem by creating the required population of Rydberg states
from molecules cooled to a temperature below 1 K by expansion in a seeded supersonic
molecular beam. The use of supersonic expansions substantially broadens the scope of
systems that can be studied. Any substance that can be volatilized can be entrained in
a seeded molecular beam, including large molecules and refractory metal clusters. In our
experiment, we excite nitric oxide (NO), cooled to less than 1 K in a supersonic expansion
of He, to form a population of photoselected Rydberg molecules. These evolve to form a
remarkably durable cold plasma of electrons and intact nitric oxide cations.
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LT11910
The world's smallest double slit
When atoms hit a molecule, they can behave just like photons passing through a double slit. Although in this case the 'slit' is the molecule itself, being only 0.1 nanometers large, the same interference characteristics arise. But if you look into details, some striking differences to the classical photon experiment can be observed.
One of most important physics experiments of all times has already been performed around 1802 when Thomas Young discovered that light passing through a double slit shows a distinct interference pattern. This proved that light can behave as a wave. Since then, other double slit experiments have shown that not only photons, but also particles have a wave character.
New measurements on helium atoms scattering at hydrogen molecules clearly show interference patterns comparable to the classical optical double slit experiment. Thus the molecule acts as a kind of double slit, on a nanometer scale. But the results also show surprising new effects, e.g. when the 'slit' is tilted. These can be explained by the quantum mechanical properties of the particles.
