Spectroscopy with Free Electron Lasers David Bernstein SASS Talk January 28 th, 2009
Overview Who am I?! What is FLASH?! The promise of Free Electron Lasers (FELs) The Trouble with Spectroscopy Sample Fabrication The Actual Experiment Data Analysis
Who am I?!
Who am I?!
Who am I?!
Who am I?!
What is FLASH?! Free election LASer at Hamburg BOGUS ACRONYM
The promise of FELs Intense pulses containing upto 10 12 photons Short temporal pulse widths, on the order of 50-100 fs Represents a 10 8 improvement in peak intensity over 3 rd generation synchrotron sources
The promise of FELs H. Chapman, et.al., Nature Physics, 2, 839 (2006)
The trouble with Spectroscopy Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy: Commonly used at synchrotrons X-rays excite a core electron into a valence state Element Specific Sensitive to bond angle and length Can probe subsystems such as spin (magnetic) and charge (electric) subsystems independently
The trouble with Spectroscopy http://unicorn.mcmaster.ca/research/stxmintro/stxm_poly3.jpg http://ssrl.slac.stanford.edu/stohr/xmcd.htm
The trouble with Spectroscopy So whats the problem? FELs are difficult to tune (sort of ) FEL radiation is produced by Self-Amplified Spontaneous Emission. This process is inherently stochastic. FEL radiation is therefore characterized by humungous fluctuations in relevant beam parameters such as energy, position, intensity and mode profile. http://hasylab.desy.de/facilities/flash/machine/how_it_ works/sase_self_amplified_spontaneous_emission/in dex_eng.html
The trouble with Spectroscopy To deal with these problems we need to capture as much information as possible on a shot-by-shot basis.
The experiment! Grating disperses the beam by 0.3-0.4eV/mm in the vertical direction We record an entire Different parts of the spectrum in each shot sample get hit with different energy photons
The experiment!
Sample Fab In order to do this, we designed a sample with large silicon nitride windows in a silicon wafer. Half of the window is covered by the metal, the other half is blank nitride. This records the I o information METAL Si 3 N 4 membrane
Sample Fab Samples consist of 100nm thick silicon nitride windows sitting on top of silicon wafers 100nm of metal (LaMnO or Aluminum) deposited via shadow mask
Analytical Challenges We need to linearize the detector We need to normalize the transmitted intensity by the incident intensity based on the information we have from a little sliver of window next to the sample.
Linearizing the Detector We recorded a run with no sample in the beamline. A gold mesh upstream of the sample chamber recorded total pulse intensity: We also have the intensity on each pixel recorded by the (very nonlinear) detection scheme consisting of a Ce:YAG crystal imaged by an intensified CCD camera. What we want to know is the true intensity incident on a given pixel,
Linearizing the Detector To do this, we use the ansatz: We then construct an error function summed over all images, and iteratively adjust our parameters, a i, until the error function reaches an acceptable value.
Linearizing the Detector
Normalizing the Data We calculate the mode intensity in the x-direction by summing over y for each value of x. Now the spectra are flat in the x-direction. We then normalize by the average value at each y point to get the full normalized spectrum.
Normalizing the Data For this to work, we can only accept images with a single mode in the x-direction. In other words, we have good shots and bad shots. GOOD SHOT BAD SHOT
Finally the spectra!
Finally more spectra!
Acknowledgements Stanford/SSRL/Pulse Yves Acremann Andreas Scherz Mark Burkhard Jo Stohr Stanford Nanofabrication Laboratory Mahnaz Mansourpour DESY/UH Bill Schlotter Martin Beye, Torben Beeck F. Sorgenfrei Annette Pietzch Wilfred Wurth A Foehlisch