2010 September

Catching proteins in liquid helium droplets

Monday, 27th September 2010Publication highlights

Mass-to-charge selected protein ions can be stored in an ion trap and then subsequently picked up by liquid helium droplets. Fig: Bierau, et al.

Scientists at the Fritz Haber Institute (FHI) in Berlin have developed a technique for catching proteins in liquid helium droplets; these molecules are an order of magnitude larger than any molecule embedded in a helium droplet to date. Their method allows for intact large mass-selected molecular ions to be studied in a gentle and cryogenic environment. The findings, which open the door to many exciting experiments applicable to a wide range of molecular species, have been published in the prestigious journal Physical Review Letters.

Of all the known elements or materials, helium is the only one that remains a liquid down to the absolute zero temperature (-273.15 C). Liquid helium has other unique and bizarre features as well, for instance, below a certain temperature (~ 2 K) liquid helium reaches a so-called “superfluid” state. In contrast to a normal fluid, a superfluid can flow with zero friction. continue reading …

(Published: Sept. 24, 2010 | Phys. Rev. Lett. 105, 133402 )

Further Information: Gert von Helden, Frauke Bierau , and Peter Kupser

Mirror-like reflection of beams of helium atoms from rough surfaces

Usually scattering of atoms from a rough surface leads to diffuse rather than mirror-like reflection, just like how a light beam scatters diffusely from a scratched mirror. However, scientists at the Fritz Haber Institute have observed a coherent (mirror-like) reflection of beams of helium atoms from microscopically rough surfaces, and have published their results in the esteemed journal Physical Review Letters.

The Berlin researchers observed the coherent reflection at grazing incidence, and identified two regimes which they attribute to different reflection mechanisms. The first regime is found for extreme grazing incidence of the helium atoms. At incidence angles as small as 1 mrad the mirror-like reflectivity from a rough surface can be as high as 40% and, for a given surface, it depends only on a single parameter: the atom’s velocity component perpendicular to the surface. This regime is explained theoretically by quantum reflection.

In contrast to this, the second regime, found for somewhat larger angles of incidence (e.g. 10 mrad), is characterized by smaller reflectivity (just 1% or less) and by a dependence of the reflectivity on both, the perpendicular and parallel components of the atom’s velocity. In this regime a general feature was observed: Independent of the actual material of the surface (e.g. glass or metal) the reflectivity is larger with increasing parallel velocity of the atoms. In other words, when the atoms are moving faster they “see” more of the surface during their reflection, thus Dr. Bum Suk Zhao and co-workers believe that the microscopic surface roughness is effectively averaged out for these measurements.

(Published: 24 Sept 2010 | Phys. Rev. Lett. 105, 133203).

Further information: Bum Suk Zhao and Wieland Schöllkopf