Heidelberg 2015 – wissenschaftliches Programm
Bereiche | Tage | Auswahl | Suche | Aktualisierungen | Downloads | Hilfe
MO: Fachverband Molekülphysik
MO 4: Cold Molecules 1
MO 4.6: Vortrag
Montag, 23. März 2015, 15:45–16:00, PH/SR106
The cross-over from free to hindered rotation in liquid helium — Luis G. Mendoza Luna1, Nagham Shiltag1, Mark J. Watkins1, •Klaus von Haeften1, Nelly Bonifaci2, and Frederic Aitken2 — 1Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK — 2G2ELab-CNRS Equipe MDE 25 Av. des Martyrs BP 166, 38042 Grenoble Cedex 9 France
The observation of free rotation of single molecules in superfluid helium droplets 20 years ago has caused considerable interest. To better understand free rotation in quantum fluids we have investigated the rotational motion of single molecules in bulk liquid helium as a function of hydrostatic pressure and temperature using fluorescence spectroscopy in the visible region. A corona discharge was employed to generate electronic excitations in liquid helium. He2* excimers formed and decayed via cascade-radiative transitions to the ground state. During their short life the excimers do not aggregate, hence, molecular interactions in bulk liquid helium can be conveniently probed. At low hydrostatic pressures well-resolved rotational lines, indicating free rotation of He2* within liquid helium, are observed. When the hydrostatic pressure is increased the rotational lines broaden until they disappear around 6 bar. The line width depends distinctly on pressure and rotational excitation and is attributed to the reduced space available to the molecule to perform rotational motion. Our results are contrasted with the findings of molecular infrared spectroscopy in helium droplets. The microscopic origin of free molecular rotation in liquid helium is discussed.