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Probing microscopic superfluidity in the bulk and in clusters of helium using non-adiabatic alignment and fluorescence spectroscopy of single molecules

Jeudi 13 juin 2013 11:00 - Duree : 1 heure
Lieu : Salle F 318 - Institut Néel, Bâtiment F, CNRS Polygone scientifique, 25 rue des martyrs, 38000 Grenoble

Orateur : Klaus VON HAEFTEN (Department of Physics and Astronomy, University of Leicester, UK)

The term ’molecular superfluidity’ refers to the gas phase-like rotational motion of foreign molecules in superfluid helium first observed by Hartmann, Miller, Toennies and Vilesov. In broad terms, the quasi-free character of the rotation is the consequence of the weak interaction with a superfluid. Spectroscopic fingerprints of the foreign molecules reflect subtle differences to pure gas phase rotations. These have been attributed to normal-fluid helium surrounding the foreign particles, but other parameters such as rotational speed play a role as well. A comprehensive understanding of molecular rotation in helium requires control of the superfluid state, the molecular interaction and rotational excitation. All our current knowledge on rotational motion of molecules in superfluid helium derives from rotational and rovibrational spectroscopy of molecules embedded in helium clusters and droplets. So far the cluster/droplet beam technique has been the only reliable method for injecting single molecules into ultra-cold helium, but limitations exist. For example, it is impossible to change the temperature and hydrostatic pressure of clusters in a supersonic beam. Also, not all molecules that would be good benchmarks for existing theories are amenable to rovibrational spectroscopy. We have addressed these issues by introducing new experimental techniques. In order to access a wider range of molecules in helium clusters and to control the degree of rotational excitation we employ non-adiabatic alignment for rotational excitation. Depending on the laser pulse intensity and duration we generate rotational wave packets whose rotational phase is interrogated after a set time delay. In a second and very different type of experiment the bulk superfluid state is controlled. Bulk experiments have not been very popular in the past because it has been almost impossible to introduce single molecules into bulk liquid helium : at the cryogenic temperatures all foreign entities normally freeze on the container walls. For this reason we use short-lived He2* excimers continuously generated in a corona discharge as molecular probes. The excimers fluoresce and we are able to resolve discrete rotational lines, even in the normal fluid phase of helium. It is therefore possible to probe the rotational interaction in different phases of helium through the varying effective moments of inertia. In all these experiments the local structure of the helium close to the molecular entities is important. To better understand the local structure and its variation with temperature and pressure we conduct mobility measurements and model the mobilities using thermodynamic state equations of the mixed system. For helium ions in helium and electrons in helium these simulations show excellent agreement with measured mobilities. Also they provide insight into the formation and compression of the boundary layers as a function of pressure, temperature and density. In my presentation, I will explain molecular superfluidity and the state of the art, I will explain the experimental techniques in detail and I will report of the current results of these activities.

Contact : frederic.aitken@grenoble.cnrs.fr

Discipline évènement : (Physique)
Entité organisatrice : (Institut Néel) -
Entité organisatrice : (LANEF)
Nature évènement : (Séminaire)
Site de l'évènement : Polygone scientifique

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