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Physics Department Colloquia

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Events During the Week of April 21st through April 28th, 2019

Monday, April 22nd, 2019

Special Colloquium
Photoinduced reaction mechanisms in molecular systems probed with elemental specificity via ultrafast X-ray spectroscopy
Time: 3:30 pm
Place: 2103 Chamberlin Hall
Speaker: Nils Huse, Hamburg University
Abstract: Understanding the interplay of structural, electronic and spin degrees of freedom is paramount for mechanistic insights into how matter transforms upon external stimuli such as light. The elemental specificity of X-ray spectroscopy provides unique complimentary information to vibrational and electronic spectroscopy or non-resonant scattering techniques when studying reaction mechanisms in chemical and materials science. This can be especially useful for metal atoms and their nearest neighbors in transition-metal complexes [1,2] or so-called heteroatoms in functional groups of organic molecules [3,4] where specific atomic sites are of particular importance for the physical and chemical properties of molecular or solid state systems. I will introduce basic concepts of X-ray absorption spectroscopy and RIXS, the X-ray analogue to resonant Raman scattering, as methods to probe electronic structure on ultrafast time scales before presenting two examples of photo-induced reactions to show how X-ray spectroscopy of ligand- and heteroatoms can (i) unravel the role of ligand-field states in transition-metal complexes and (ii) identify transient excited states and new species in sulfur-containing molecules.

[1] B. Van Kuiken et al., J. Phys. Chem. Lett. 7, 465 (2016)
[2] A. A. Cordones et al., Nat. Comm. 9, 1989 (2018)
[3] M. Ochmann et al., J. Am. Chem. Soc. 139, 4789 (2017)
[4] M. Ochmann et al., J. Am. Chem. Soc. 140, 6554 (2018)
Host: Matt Herndon
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Tuesday, April 23rd, 2019

No events scheduled

Wednesday, April 24th, 2019

No events scheduled

Thursday, April 25th, 2019

No events scheduled

Friday, April 26th, 2019

New Science Opportunities at X-ray Free Electron Lasers
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Uwe Bergmann, Stanford PULSE Institute, SLAC National Accelerator Laboratory
Abstract: Over the past century X-rays have revolutionized numerous fields of science. Starting in the 1970s powerful new synchrotrons sources have dramatically advanced the scientific use of X-rays. In the last decade new X-ray free electron lasers, such as the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory, have come to light. These coherent X-ray sources produce ultra-short pulses with a brightness that is ten billion times larger than synchrotron sources. For the first time scientist can study matter not just at the length scale of atoms and molecules, but also at the time scale of molecular motion. We will describe these machines and present some examples of recent X-ray laser research.

One example is our recent work on stimulated x-ray emission spectroscopy. Here, we have created inner shell x-ray fluorescence laser signals from manganese compounds. To achieve this exponential signal gain, we used highly focused ultrashort x-ray pump pulses from LCLS. The observed signal consisted of very short x-ray pulses up to 10^6 times stronger than normal x-ray fluorescence. Remarkably, these x-ray pulses preserved the small differences in fluorescence energy, caused by the different chemical surrounding of the manganese ions. We describe the phenomena and how we plan to apply this and similar approaches to learn more about transition metal systems and their function in catalysis, chemical, and materials science.
Host: Mark Eriksson
Video: https://vod.physics.wisc.edu/media/2019_04_26.m4v
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