More Like this

1. Toward Å–fs–meV resolution in electron microscopy: systematic simulation of the temporal spread of single-electron packets

   https://doi.org/10.1039/d1cp03518e  

   Curtis, Wyatt A. ; Flannigan, David J. ( October 2021 , Physical Chemistry Chemical Physics)

   Though efforts to improve the temporal resolution of transmission electron microscopes (TEMs) have waxed and waned for decades, with relatively recent advances routinely reaching sub-picosecond scales, fundamental and practical challenges have hindered the advance of combined Å–fs–meV resolutions, particularly for core-loss spectroscopy and real-space imaging. This is due in no small part to the complexity of the approach required to access timescales upon which electrons, atoms, molecules, and materials first begin to respond and transform – attoseconds to picoseconds. Here we present part of a larger effort devoted to systematically mapping the instrument parameter space of a TEM modified to reach ultrafast timescales. With General Particle Tracer, we studied the statistical temporal distributions of single-electron packets as a function of various fs pulsed-laser parameters and electron-gun configurations and fields for the exact architecture and dimensions of a Thermo Fisher Tecnai Femto ultrafast electron microscope. We focused on easily-adjustable parameters, such as laser pulse duration, laser spot size, photon energy, Wehnelt aperture diameter, and photocathode size. In addition to establishing trends and dispersion behaviors, we identify regimes within which packet duration can be 100s of fs and approach the 300 fs laser limit employed here. Overall, the results provide a detailedmore »picture of the temporal behavior of single-electron packets in the Tecnai Femto gun region, forming the initial contribution of a larger effort.« less
2. Simulating the Upper Hybrid Instability as a Cause for 150 km Echoes

   https://doi.org/10.1029/2022JA031223  

   Green, A. ; Oppenheim, M. ; Longley, W. ( April 2023 , Journal of Geophysical Research: Space Physics)

   One Hundred and Fifty kilometer echoes are a type of strong radar echo observed in the valley region of the equatorial ionosphere, whose origin was long standing mystery. Recently, a new upper hybrid (UH) instability theory, driven by high energy photoelectrons, has been proposed to explain most features of 150 km echoes. However, this instability excites high frequency electron modes, whereas radars observe low frequency ion modes. To explain 150 km echoes, the UH instability must ultimately excite ion acoustic modes. This paper describes a set of particle‐in‐cell simulations used to study how photoelectrons interacting with a cold background plasma generate UH waves, and how these drive ion acoustic waves measured by radars. We implement a new electron‐N₂collision algorithm to better model the bump on tail features of the photoelectron distribution in the valley region. These simulations show that photoelectrons drive unstable UH waves that strongly enhance ion acoustic waves. We also show a strong frequency dependence on power in the ion line, which explains observational differences between radars, most notably the lack of echoes at ALTAIR. The photoelectron driven UH instability successfully reproduces most features of 150 km echoes associated with naturally enhanced incoherent scattering.
3. Global perspectives of the bulk electronic structure of URu ₂ Si ₂ from angle-resolved photoemission

   https://doi.org/10.1088/2516-1075/ac4315  

   Denlinger, J. D. ; Kang, J-S ; Dudy, L. ; Allen, J. W. ; Kim, Kyoo ; Shim, J-H ; Haule, K. ; Sarrao, J. L. ; Butch, N. P. ; Maple, M. B. ( January 2022 , Electronic Structure)

   Previous high-resolution angle-resolved photoemission (ARPES) studies of URu₂Si₂have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level (E_F) at low photon energies near the zone center, with an emphasis on electronic reconstruction due to Brillouin zone folding. A substantial challenge to a proper description is that these states interact with other hole-band states that are generally absent from bulk-sensitive soft x-ray ARPES measurements. Here we provide a more globalk-space context for the presence of such states and their relation to the bulk Fermi surface (FS) topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure using photon energies intermediate between the low-energy regime and the high-energy soft x-ray regime. Small-spot spatial dependence,f-resonant photoemission, Si 2pcore-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of theE_F-crossing bands and their relation to specific U- or Si-terminations of the cleaved surface. The bulk FS topology is critically compared to density functional theory (DFT) and to dynamical mean field theory calculations. In addition to clarifying some aspects of the previously measured high symmetry Γ,ZandXpoints, incommensurate 0.6a* nested Fermi-edge states located alongZ–N–Zare found to be distinctlymore »different from the DFT FS prediction. The temperature evolution of these states aboveT_HO, combined with a more detailed theoretical investigation of this region, suggests a key role of theN-point in the hidden order transition.

   « less
4. A localized view on molecular dissociation via electron-ion partial covariance

   https://doi.org/10.1038/s42004-022-00656-w  

   Allum, Felix ; Music, Valerija ; Inhester, Ludger ; Boll, Rebecca ; Erk, Benjamin ; Schmidt, Philipp ; Baumann, Thomas M. ; Brenner, Günter ; Burt, Michael ; Demekhin, Philipp V. ; et al ( December 2022 , Communications Chemistry)

   Abstract Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d 3/2 and 4d 5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.
5. Low Repetition-Rate, High-Resolution Femtosecond Transmission Electron Microscopy

   https://doi.org/10.1063/5.0128109  

   Flannigan, David J. ; Curtis, Wyatt A. ; VandenBussche, Elisah J. ; Zhang, Yichao ( October 2022 , The Journal of Chemical Physics)

   Spatial and energy resolutions of state-of-the-art transmission electron microscopes (TEMs) have surpassed 50 pm and 5 meV. However, with respect to the time domain, even the fastest detectors combined with the brightest sources may only be able to reach the microsecond timescale. Thus, conventional methods are incapable of resolving myriad fundamental ultrafast ( i.e., attosecond to picosecond) atomic-scale dynamics. The successful demonstration of femtosecond (fs) laser-based (LB) ultrafast transmission electron microscopy (UEM) nearly 20 years ago provided a means to span this nearly 10-order-of-magnitude temporal gap. While nanometer-picosecond UEM studies of dynamics are now well established, ultrafast Å-scale imaging has gone largely unrealized. Further, while instrument development has rightly been an emphasis, and while new modalities and uses of pulsed-beam TEM continue to emerge, the overall chemical and materials application space has been only modestly explored to date. In this Perspectives article, we argue that these apparent shortfalls can be attributed to a simple lack of data and detail. We speculate that present work and continued growth of the field will ultimately lead to the realization that Å-scale fs dynamics can indeed be imaged with minimally modified UEM instrumentation and with repetition rates ( f rep ) below - andmore »perhaps even well below - 1 MHz. We further argue that use of low f rep , whether for LB UEM or for chopped/bunched beams, significantly expands the accessible application space. This calls for systematically establishing modality-specific limits so that especially promising technologies can be pursued, thus ultimately facilitating broader adoption as individual instrument capabilities expand.« less