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Transversely Coherent Helium Atom Beams

Primary Collaborators:  Dan DePonte, Forest Patton, University of Oregon

Greg Elliott, University of Puget Sound

An important feature of many classes of materials is that the energetic interactions that determine their thermodynamic and mechanical properties arise from weak forces that operate on the nanometer length scale.  These interactions nonetheless manifest themselves on a macroscopic scale in ways that lead to unusual and useful properties.  These statements apply as well to superconductors and ferromagnets as they do to complex fluids and biological materials.  Despite the many spectacular advances made in developing new microscopy, spectroscopy, and scattering techniques, a mechanistic understanding of this microscopic-macroscopic connection has not been achieved in many cases.  Part of the reason for this is that most techniques do not provide simultaneous spatial and dynamical information on key length and time scales.  Diverse phenomena that involve, for example, thermal activation or exotic phase separation, can only be partially studied at present because the important microscopic modes are characterized by nanometer length scales and microsecond time scales - a regime that is not well-covered by existing experimental techniques.

An approach to address this issue currently being developed in our laboratory entails a new application of supersonic atomic helium and molecular hydrogen beams.  Specifically, our goal is to produce a transversely coherent beam of sufficient intensity that the atomic or molecular analog of dynamic light scattering can be performed on important length and time scales.  Simply stated, this means that we will collimate a beam of helium atoms such that the transverse product of spatial and momentum dimensions is comparable to Planck's constant.  For this reason, as is the case for the coherent soft x-ray experiments described elsewhere on this web site, the ‘brightness’ of our helium source [atoms/(s x solid angle x unit area)] is the key figure of merit.  We have shown that the brightness of a free jet helium expansion is only 1-2 orders of magnitude less than that of an undulator at a third generation synchrotron radiation facility.  Such a beam will provide an attractive probe for studying complex interfacial phenomena since helium atoms achieve the ultimate in surface sensitivity (much higher than x-rays, for example), they are non-invasive and non-damaging, and they can provide useful topographic and spectroscopic contrast. 

Our apparatus, a schematic of which is shown above, is presently being commissioned, with support from an NSF Major Research Instrumentation Grant and the Murdock Charitable Trust.  We have produced the first pinhole diffraction pattern for thermal helium atoms and hydrogen molecules and a speckle/diffraction pattern of an irregularly shaped aperture plate.  We are investigating the possibility of inverting the latter using iterative phase retrieval techniques.