Diffraction of Cluster Beams from Nanoscale Transmission Gratings

Abstract

Since van der Waals clusters produced in molecular beam expansions have a broad size distribution, experimental techniques are needed to select clusters of well defined sizes. A size selective detection of clusters is not possible with conventional mass spectrometers because of cluster fragmentation and dissociation in the ionization process. Recently our group has demonstrated that it is possible to select and analyze light clusters by diffracting them from a free-standing nanostructured transmission grating [1, 2], Since this technique depends on the wave nature of the particles it has the advantage of being essentially non-destructive. This feature has been demonstrated with small clusters of ⁴He and allowed for the unambiguous detection of He₂ and He₃ [1, 2]. With a predicted binding energy of only 1.3mK (= 1.1 · 10^-7eV) and a mean nuclear distance of about 55 [3] the helium dimer is thought to be the weakest bound diatomic molecule in the ground state. The bond is sufficiently weak to support an Efimov state in He₃ [4]. Efimov states are special long range three particle bound states which occur only if the corresponding two particle system is sufficiently weakly bound.