Helium Nanodroplet Experiments

 

Spectroscopy of Molecules and Clusters inside Helium Droplets

 

 

The helium isotopes 4He and 3He are the only substances which remain liquid down to the lowest temperatures and exhibit the phenomenon of superfluidity below 2.2 K (4He), and below 3·10-3K (3He). Superfluidtiy exhibits many fascinating macroscopic manifestations such as

  1. flow without resistance,
  2. a vanishing viscosity,
  3. the ability to “creep” out of vessels against the force of gravity,
  4. the fountain effect which is driven by a type of Maxwell demon which separates the superfluid from the normal fluid components, and
  5. an enormous heat conductivity which is 30 times greater than that of copper.
Apart from experiments carried out more than 20 years ago involving the frictionless drift of ions at velocities less than a critical velocity of 58 m/s there was up to recently no evidence for microscopic manifestations of superfluidity.

 

In 1990 our group demonstrated that 4He droplets produced in a free jet expansion could readily pick up foreign atoms either singly or as clusters. Then in 1992 Scoles and coworkers at Princeton detected unexpected sharp spectral features in the infrared absorption of embedded sulphur hexafluoride (SF6) molecules. In our laboratory in 1994 Ralf Fröchtenicht and Andrej Vilesov quite unexpectedly observed the P-, Q- and R-branches of SF6 and subsequently other members of the group were able to resolve the individual rotational lines of SF6 and later of OCS in helium droplets.

 

 

Experimental Setup

In the apparatus the pure 4He droplets produced in the expansion of the gas from a cryogenically cooled source pass through a scattering chamber containing the gas of molecules to be investigated. Depending on the scattering chamber pressure, single molecules - or several molecules which coalesce to form clusters in the interior - are picked up. The method can be readily applied to fragile biomolecules or refractory metals since vapor pressures of only 10-6 – 10-5 mbar which are accessible at moderate scattering chamber temperatures are required for embedding. For spectroscopic interrogation the laser beam is introduced coaxially from the far end of the apparatus. Resonant absorption of the photons is detected by the depletion of the mass spectrometer signal. At resonance the droplet is momentarily heated leading to the evaporation of a significant fraction of its atoms and the ionization cross section is reduced correspondingly. Subsequent spectroscopic studies of embedded glyoxal (C2H2O2) molecules in the visible provided evidence that the 4He droplets were superfluid.

 

 

Results

The above comparison of the rotationally resolved infra-red spectra of the free OCS molecules, part (a), with the OCS molecules embedded in a 4He droplet, part (b), and in a 3He droplet, part (c), provides an overview of the new phenomenon. The closer spacing of the lines in the 4He droplets, part (b), is attributed to a solvation layer of non-superfluid helium which, since it adiabatically follows the rotations of the chromophore, explains the increase in the moment of the inertia and reduction of the rotational energy constant. The lack of a Q- branch in the same droplet spectrum indicates, surprisingly, that the molecule with its solvation layer has the same symmetry as the free molecule. Moreover, from the relative line intensities in the droplet spectrum, which closely follows a Boltzmann distribution, a rotational temperature of 0.37 K could be determined. Since this is the same temperature as predicted for the translational degrees of freedom inside the droplets this confirms that the molecules are fully thermally equilibrated. Finally the broad unresolved spectrum measured in non-superfluid 3He droplets, part (c), which from other studies are known to be even colder, at 0.15 K, demonstrates that the free rotations found in the 4He droplets must be related to their being superfluid. These experiments were crucial in identifying the phenomenon of free rotations as a new manifestation of microscopic superfluidity. They have also demonstrated the enormous potential of 4He droplets as a uniquely cold and gentle cryomatrix.

 

This area of research is now being pursued in more than 20 laboratories worldwide and since the early 1990’s has lead to well over 1000 publications. In addition to high resolution spectroscopic studies of single biomolecules and taylor-made or self organized clusters current research is directed at studying molecular dynamic processes such as photoionization, photoelectron spectroscopy and photodissociation inside helium droplets. Another current research direction is the study of photon induced isomerization and tautomerization as well as bimolecular reactions. The synthesis of large metal and semiconductor clusters and their soft landing on metal surfaces is another emerging area of helium droplet research.

 


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last revision:    E. Hulpke, October 2007