Soft X-ray Dichroism Absorption Microscopy and Spectroscopy at the SSRL
X-ray absorption spectroscopy (XAS)
utilizes the energy dependent absorption of x-rays to obtain information about
the elemental composition of the sample and the chemical environment of each
element (see also NEXAFS Spectroscopy). Typically XAS is measured by
detecting the intensity of an x-ray beam before and after it is transmitted
through a sample. This approach provides information about bulk properties.
However, one can also achieve excellent sensitivity to interfaces, buried
layers and diluted impurities or dopants by detecting electrons or photons that
are emitted from the sample due to secondary processes instead. Finally, one
can employ the polarization dependence of the x-ray absorption process
(dichroism) to obtain information about low symmetry charge distribution or
long range magnetic order. For example ferromagnetic materials like Cobalt will
exhibit X-Ray Magnetic Circular Dichroism (XMCD) while antiferromagnetic materials
like Nickeloxid will show X-Ray Magnetic Linear Dichroism (XMLD). By using
microscopes that are capable of imaging the spatial variation of the X-ray
absorption one can finally obtain images with chemical, elemental or magnetic
contrast as shown in the figure on the left.
SPEAR3 is a third generation
synchrotron and therefore an excellent source of highly brilliant soft x-rays
beams produced by state of the art insertion devices in the straight sections
of the synchrotron. Since it is important for most soft x-ray dichroism
experiments to have full control over the incident polarization of the incoming
x-rays it is necessary to use so called elliptical polarizing undulators as a
source. These devices also provide coherent x-rays, which makes it also
possible to also use holography like imaging techniques like Speckle.
The elliptical
polarizing undulator (EPU) beamline 13 provides soft x-rays with variable
polarization for dichroism spectroscopy, microscopy, and imaging on
magnetic materials. Branchline 13-3
(contact: Andreas
Scherz) currently features a setup for coherent imaging experiments.
Branchline 13-1 (contact: Hendrik
Ohldag) is under constructions and will be available at the end of 2009 equipped
with a scanning transmission x-ray microscope (STXM) and a general purpose
x-ray absorption spectroscopy apparatus for XMCD and XMLD measurements.
