A monochromator is a device that uses a dispersive optic like a diffraction grating to deliver a single wavelength of light to an exit slit. [A spectrometer differs from a monochromator in that a spectrometer gives you the intensity as a function of wavelength over some range (ie. the rainbow), whereas a monochromator might give you just red light, for example.] In order to image functional groups in organic specimens, it is useful to be able to tune the energy of the light over a specified range to measure the optical response of a sample. At the X1A beamline at Brookhaven National Laboratory, the light exiting the monochromator is imaged by a Fresnel zone plate to produce a small focal spot through which the specimen is scanned. By first scanning pixel-by-pixel, and then repeating this scan at each of a series of photon energies, one can build up a measurement of the spectral response of each point in the specimen.

Monochromators tend to be optimized over a certain photon energy range based on their geometry. Using the Center for Environmental Molecular Science (CEMS) support, the X1A2 monochromator was modified to incorporate a second exit slit to deliver improved energy resolution at the Iron L edge. My project is to understand the role of this modification, characterize its performance, and demonstrate the improved capabilities of the system by looking at a number of iron compounds relevant to environmental science.

The first step in my project is reading materials on the Rowland circle condition for grating monochromators and understanding the basic properties of their performance. From there I will be documenting and verifying a computer program that is meant to calculate the energy resolution of the X1A2 monochromator. [The original author of this program (Barry Winn, a former PhD student) did not describe its calculational steps very well, and the x-ray microscopy group would like to gain confidence that its calculations are correct.] Once that has been achieved, I will measure the energy resolution of the X1A2 at both the Oxygen K edge and Iron L edge with both of the two monochromator exit slit settings. As a fourth and final step, I will attempt to prepare a number of iron oxide samples and build up a library of their spectral responses as a reference for future environmental science work.