Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session D02: Ultrafast XUV/Soft X-ray SpectroscopyFocus Session Recordings Available
|
Hide Abstracts |
Chair: Matthias Werner, Qilimanjaro Quantum Tech Room: McCormick Place W-175C |
Monday, March 14, 2022 3:00PM - 3:36PM |
D02.00001: Uncovering new materials behaviors using novel X-ray and electron computational imaging Invited Speaker: Margaret M Murnane Nanoscale imaging science is undergoing a revolution, driven by advances in coherent light sources, electron sources, algorithms and detectors. New imaging tools and methods can now capture multi-scale atomic/electronic/chemical/magnetic/structural orders and the functional properties of materials, from Ångstrom to centimeters and attosecond to static. This can uniquely connect structure to function. This talk will highlight several new discoveries including: the first general method to map 3D topological spin textures and their interactions using soft x-ray vector ptychography; the ability to directionally channel of heat by smart geometric design — a novel materials phenomenon that is associated with increased phonon scattering in the transverse plane, that drives enhanced heat conduction into the substrate; the ability to directly observe the motion of atoms during annealing of nanoparticles, leading to better understanding of this important materials process; the ability to map defects at the sub-atomic level in 2D materials; and the ability to delicately manipulate strong-coupled quantum materials using light.[1-4]
|
Monday, March 14, 2022 3:36PM - 4:12PM |
D02.00002: Direct Time- and Momentum-Resolved Imaging of Exciton Dynamics in Monolayer WS2 Invited Speaker: Thomas K Allison Monolayer transition metal dichalcogenides (TMDs) have generated immense interest in recent years for their potential in a variety of optoelectronic applications due to their unique spin-valley optical selection rules. The remarkable properties of these materials are governed by strongly bound excitons that exhibit a variety of complex radiative and non-radiative relaxation pathways. In this talk, I will first show how time- and angle-resolved photoemission spectroscopy can be implemented to efficiently image, in parallel, the ultrafast dynamics of the full Brillouin zone of micron-sized TMD heterostructures. With a repetition rate of 61 MHz, we are able to employ low, few μJ/cm2 pump fluences to maintain low exciton densities and determine the lifetimes and relaxation pathways of both optically bright and momentum- and/or spin-forbidden dark excitons with 225 fs time resolution. I will then present our recent experimental results directly imaging the formation and relaxation dynamics of A and B excitons in monolayer WS2. Following linearly polarized pump excitation, we observe the relaxation of hot excitons within the K valleys to form a long-lived dark exciton state, as well as population of the Sigma valleys yielding long-lived momentum-forbidden excitons. With circularly polarized photoexcitation, we observe the initial preparation of excitons in the K valleys and a delayed rise for signal in the K’ valleys, indicative of valley depolarization due to intervalley scattering. |
Monday, March 14, 2022 4:12PM - 4:24PM |
D02.00003: Combining soft X-ray spectroscopy and reflectivity with DFT for optical models of polarized RSoXS to reveal molecular alignment in nanostructures Victor M Murcia, Brian A Collins Resonant Soft X-Ray Reflectivity (XRR) and Near Edge X-ray Absorption Fine Structure spectroscopy (XAS) are powerful techniques that can be used for depth profiling, orientational analysis and determination of optical constants. Polarized Resonant Soft X-ray Scattering (RSoXS) is uniquely sensitive to local molecular orientation regardless of crystallinity, making it a powerful tool in characterizing various types of nanostructures. Building block models (BBM), used to measure global orientation in XAS won’t work for RSoXS as it assesses differences in local ordering and means that XAS measurements alone will not provide enough information for an optical model. We have developed an optical model that combines angle-dependent XAS with density functional theory (DFT) to algorithmically create an optical tensor for molecules. We first show how the algorithm combines clustering methods to generate a BBM that can be used to quantitively fit the XAS of Copper(II)Phthalocyanine(CuPc) and subsequently generate optical constants. Then, we use these optical constants to fit the scattering anisotropy of CuPc films as measured by RSoXS. Finally, we show how this model can also be used to carry out depth profiling and orientational analysis on CuPc films via XRR. |
Monday, March 14, 2022 4:24PM - 4:36PM |
D02.00004: Ultrafast catalytic dynamics unveiled with UV pump - X-Ray probe experiment and simulations Han Wang, Alan C Luntz, Johannes Voss Understanding of charge transfer and chemical bond breaking processes is key to gaining a deeper insight into catalytic activity and selectivity and identifying new catalyst materials. With the development of X-ray free-electron laser (XFEL) facilities around the world, X-ray spectroscopy in pump-probe experiments will be an essential tool for unveiling unprecedented details of heterogeneous catalytic reactions with femtosecond time resolution. A femtosecond UV pump laser will excite the system and initiate the charge transfer and bond breaking process, and the absorption spectra of the probe X-ray laser pulses record the dynamics. Due to the large amount of information in the X-ray absorption spectra (XAS) and the vast amount of competing reaction pathways, it is a huge challenge to discriminate the many potentially observed reaction intermediates from the spectra. Our theoretical calculations simulate the charge transfer dynamics and provide the X-ray absorption spectra for each snapshot of the chemical reaction, which is comparable to the transient XAS recorded in the experiment. As an example, we present here the charge transfer dynamics between a transition metal catalyst and graphene. Our simulation method will provide detailed dynamics for the temporal evolution of surface reactions for a wide range of catalytic processes in future studies. |
Monday, March 14, 2022 4:36PM - 4:48PM |
D02.00005: Core-Valence Attosecond Transient Absorption Spectroscopy of Polyatomic Molecules Nikolay Golubev, Alexander I Kuleff, Jiri J Vanicek Tracing ultrafast processes induced by interaction of light with matter is often very challenging. In molecular systems, the initially created electronic coherence becomes damped by the slow nuclear rearrangement on a femtosecond timescale which makes real-time observations of electron dynamics in molecules particularly difficult. In this work, we report an extension of the theory underlying the attosecond transient absorption spectroscopy (ATAS) for the case of molecules, including a full account for the coupled electron-nuclear dynamics in the initially created wave packet, and apply it to probe the oscillations of the positive charge created after outer-valence ionization of the propiolic acid molecule. By taking advantage of element-specific core-to-valence transitions induced by x-ray radiation, we show that the resolution of ATAS makes it possible to trace the dynamics of electron density with atomic spatial resolution. |
Monday, March 14, 2022 4:48PM - 5:24PM |
D02.00006: Using X-rays to probe excited state structural and electronic dynamics: Cobalamins as a case study Invited Speaker: Roseanne J Sension The fate of a photoactive molecule is determined by the electronic and structural rearrangements that follow excitation. Femtosecond (fs) X-ray free electron lasers (XFELs) have made it possible to use X-ray absorption spectroscopy to probe changes in electronic configuration and atomic structure as a function of time, beginning from the initial excited state. Hard X-rays permit the investigation at the Co K-edge focused on structural changes, soft X-rays at the Co L-edge and N K-edge permit more detailed investigation of the evolution of the electronic structure at the photoactive Co center. Femtosecond X-ray absorption near edge structure (XANES) and X-ray Emission spectroscopy (XES) at the Co K-edge to characterize the excited state dynamics of cobalamins, B12 coenzymes and analogues. Time-resolved measurements in the pre-edge region and steady state measurements in the soft X-ray region lay the ground work for femtosecond measurements using the soft X-ray liquid jet end stations being developed at both LCLS and the EuXFEL. |
Monday, March 14, 2022 5:24PM - 6:00PM |
D02.00007: Visualizing Electron Motion at Photochemical Interfaces Using Ultrafast XUV Spectroscopy Invited Speaker: Robert Baker Directly observing electron dynamics at surfaces is required to understand and control the material properties that determine efficiency of many applications including efficient energy conversion as well as ultrafast information processing. Toward this goal, we have developed extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy as a surface-specific analog of XUV transient absorption. This method combines the benefits of traditional X-ray absorption spectroscopy, such as element, oxidation, and spin state resolution, with surface sensitivity and ultrafast time resolution. Using this technique, we investigate charge and spin dynamics in materials with applications ranging from photocatalysis to optical control of magnetic switching. In one example, we describe a systematic comparison of surface and bulk electron polaron formation in hematite showing that surface self-trapping dynamics differ significantly from bulk and that these dynamics can be systematically tuned by surface molecular functionalization offering the possibility for design of photocatalytic interfaces with enhanced carrier transport based on earth abundant materials. In a second example, we highlight evolving applications of XUV-RA spectroscopy to study spin dynamics at surfaces. Applications include understanding ultrafast spin crossover in magnetic semiconductors as well as control of spin polarized electron dynamics at chiral photochemical interfaces. Last, I will describe capabilities that will soon become available at the NSF National eXtreme Ultrafast Science Facility (NeXUS) that is currently under development at Ohio State University. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2025 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700