Helium Clusters

 

DIFFRACTION OF ATOMS, MOLECULES AND CLUSTERS FROM NANO-TRANSMISSION GRATINGS

Diffraction of small helium clusters from transmission gratings

Quantum mechanics postulates the wave nature of atoms and molecules. Therefore phenomena known from experiments involving the wave nature of visible light, like diffraction and interference, can be observed in similar experiments with these particles. The wavelength of matter waves, λ, is determined according to de Broglie (1924) by the momentum, p = mv, of the respective particle with mass m and velocity v

λ = h/p,

where h is Planck's constant.

In this project, a molecular beam containing clusters is diffracted from an extremely fine structured transmission grating. On cooling the source of a helium nozzle beam to sufficiently low temperatures, the beam will contain, in addition to the atoms, helium clusters of various sizes. Due to collisions in the expansion, the clusters will achieve the same narrow velocity (Δv/v ≤ 1 %) as the atoms in the beam and their de Broglie wavelengths will therefore be directly proportional to the cluster mass. The beam is collimated by 5 micron wide slits assuring a large lateral coherence. The atoms and clusters diffracted from the grating are then detected by a sensitive mass spectrometer which rotates with respect to the grating.



The "heart" of this apparatus is the free standing nano-structured SiNx grating which comes on a chip with three grating windows. The bars and slits are typically about 50 nm (500 Å) and the period is 100 nm (1000 Å). We are very grateful to Dr. Tim Savas and Prof. Hank Smith of the Department of Electrical Engineering at MIT for providing



us with these gratings. As far as we are aware these free standing gratings have the smallest periods available world wide at present.

A typical diffraction pattern at a source temperature of 40.5 K and a source pressure of 50 bar displays a large number of different peaks with intensities varying over several orders of magnitude.



The maxima are marked according to the number of atoms in the cluster: 1= atom, 2 = dimer, 3 = trimer etc. The peaks with the same numbers correspond to different diffraction orders, where the peak at the smallest angle corresponds to the first order, etc. Well resolved diffraction patterns have been masured for He, H2, D2, Ne, Ar, Kr, Xe, CH3F and CH3H as well as for metastable He and Ne atoms and in many cases for the corresponding clusters.

Especially for studies of weakly bound clusters this technique has the unique advantage of being non-destructive. In particular it is the first technique to directly prove the existence of the neutral helium dimer. From the relative intensities of the dimer diffraction peaks out to high orders it was possible to determine the average separation of the atoms in the dimer to be 52 Å making it the largest diatomic molecule in the ground state that is found in nature. The He-dimer, like the deuteron, can be classified as a "halo" composite particle for which the size depends on the binding energy. In this way the binding energy was determined from the measured size to be 1.1·10-7 eV or 1.3 milli-Kelvin.
Early references:
[1] W. Schöllkopf and J.P. Toennies, Science 266, 1345 (1994)
[2] W. Schöllkopf and J.P. Toennies, J. Chem. Phys. 104, 1155 (1996)
[3] T.A. Savas et al., J. Vac. Sci. Technol. B 13, 2732 (1995)
For recent publications (from 2004 to Sept. 2007) on atom optics from this group please click here

 

 

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last revision:   E. Hulpke, October 2007