A team of researchers working at SSRL Beam Line 13-3 have devised an imaging technique that combines methods from traditional x-ray crystallography and x-ray holography, circumventing one of the major technical hurdles associated with capturing detailed images of non-periodic structures. The results were published in the August 15 edition of Physical Review Letters.

X-ray diffraction has been widely used to determine the structure of macromolecules like proteins, but samples must first be grown into a crystal where the molecules are arranged in a periodic pattern. The regular ordering of the molecules in the sample makes it much easier to recover information about the phase of the diffracted x-rays, which then enables researchers to recreate the structure of the molecules in the sample.

The desire to image all kinds of natural and artificial nanostructures or materials that exhibit nanoscale ordering has led to the development of x-ray imaging techniques that do not rely on any form of sample periodicity. One such method is real space x-ray microscopy by use of suitable x-ray lenses. Other "lensless" methods use coherent x-ray scattering, however the reciprocal space "speckle" pattern needs to be inverted into real space. This step is impeded by the famous "phase problem", which in the past has been overcome by MAD techniques or holographic methods. The SSRL researchers found a new way to solve the phase problem by capturing two scattering patterns of a sample-in this case, microscopic polystyrene beads on a specially prepared thin film-using coherent x-rays of two different energies. A "resonant" beam at the carbon K edge (285 eV) was used obtain a diffraction pattern that highlighted the carbon atoms in the sample, along with a "non-resonant" beam used to capture a baseline diffraction pattern as a reference. The two images were then combined, allowing the researchers to recover the phase information and recreate a two-dimensional image of the sample. The ability to capture detailed images of non-periodic structures holds great promise for imaging all kinds of nanostructures which in most cases are not periodic.

A. Scherz, D. Zhu, R. Rick, W. F. Schlotter, S. Roy, J. Lüning, J. Stöhr, "Nanoscale Imaging with Resonant Coherent X Rays: Extension of Multiple-Wavelength Anomalous Diffraction to Nonperiodic Structures", Phys. Rev. Lett. 101, 076101 (2008).

To learn more about this research see the full scientific highlight at:
http://www- ssrl.slac.stanford.edu/research/highlights_archive/lensless_MAD.html