Capture and ionization of atoms and molecules in liquid helium droplets

David M. Ceperley

What is the minimum number of atoms needed for superfluidity? According to the results of a remarkable molecular spectroscopy experiment, there is definite proof that a system of only 60 atoms can be superfluid. Slava Grebencv and colleagues at the Max Planck Institute for Fluid Dynamics in Göttingen, Germany, found that small molecules dissolved in droplets of liquid helium can rotate freely, just like they do in a vacuum (S Grebencv, J P Tocnnies and A Vilesov 1998 Science 279 2083). The molecules rotate freely because the surrounding helium is a "superfluid" – a liquid without viscosity. In effect, the liquid helium acts as a vacuum.

Lei Chen

Siu A. Chin, E. Krotscheck

Felix Laimer, Fábio Zappa, Elisabeth Gruber +1 more

A beam of size-selected charged helium droplets was crossed with an electron beam, and the ion efficiency curves for the product droplets in all different charge states were recorded. We estimate that the selected helium droplets on their passage through the electron beam are hit by several hundred electrons which can interact with the individual He atoms of the droplets. Reaction channels corresponding to the removal or capture of up to eight electrons were identified, and in all cases, inelastic scattering and the formation of metastable helium played a significant role.

Andreas Mauracher, Stefan Ernest Huber

Felix Laimer, Fábio Zappa, P. Scheier +1 more

Abstract The detection of multiply charged helium droplet anions is reported for the first time. By ionizing droplets of superfluid helium with low energy electrons (up to 25 eV), it was possible to produce droplets containing up to five negative charges, which remain intact on the timescale of the experiment. The appearance sizes for different charge states are determined and are found to be orders of magnitude larger than for the equivalent cationic droplets, starting at 4 million He atoms for dianions. Droplets with He*− as charge carriers show signs of being metastable, but this effect is quenched by the pickup of water molecules.