Magnetic Materials

We investigate the influence of the x-ray pulse intensity on x-ray spectra measured in absorption or fluorescence. The high peak intensity of X-ray FELs can have a dramatic influence on the electronic structure of the sample. As electron dynamics involves processes on the femtosecond time scale, it is expected that the x-ray probe pulse itself leads to modification of a measured x-ray absorption and fluorescence spectrum. It is essential to test the limits of the nun-perturbative probe regime.

In collaboration with the group of Prof. W. Wurth and A. Foehlisch at FLASH, Hamburg, we demonstrated that x-ray absorption spectroscopy is feasible and have developed methods to avoid artifacts caused by the large fluctuations of the FEL beam parameters.

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Fig: a) Top view of the experimental setup. The FEL beam is diffracted from the monochromator grating. The sample (fabricated on a silicon nitride membrane) provides an area covered by the sample material as well as an area used to measure a reference spectrum. Imaging the transmitted beam, sample and reference spectra can be measured simultaneously.

To measure reliable spectra using the FEL beam, we need to record a spectrum in a single shot and simultaneously record a reference spectrum of the incoming beam. The final spectrum is the average of the single shot spectra normalized by the single shot reference spectra. One way how this can be achieved is by mounting a sample before the exit slit of a monochromator. The sample consists of two areas. One area is covered with the sample to be measured. The second area serves to provide the reference spectrum. The transmitted beam has been imaged with a 2D detector (YAG crystal + CCD camera), showing the sample and reference spectrum.