Ultrafast 3-D X-Ray Imaging of Complex Systems Achieved at Near Atomic Resolution

Imaging complicated systems in three-dimensions (3D) with near-atomic resolution on ultrafast timescales using extremely intense X-ray free-electron laser (XFEL) pulses is now becoming a reality.

One significant step toward ultrafast imaging of samples with a single X-ray shot is determining the interaction of extremely brilliant and intense X-ray pulses with the sample, including ionization rates. Scientists from the US Department of Energy’s Argonne National Laboratory (Argonne, IL, USA) and SLAC US National Accelerator Laboratory (Menlo Park, CA, USA) have developed an extended Monte Carlo computational scheme that for the first time includes bound-bound resonant excitations that drastically enhance ionization rates and can lead to an unpredictably high level of electron stripping.

The extended computation scheme addresses a daunting challenge for the standard rate equation approach, managing the exponentially large number of electron configurations that can occur when one or more excitations occur. The scheme computes atomic data only on demand, that is, when a specific electronic configuration is accessed. “This strategy allows for a natural and effective way to identify the most probable path through the quadrillions of electronic configurations to the final state,” Argonne distinguished fellow Linda Young said.

With the extended Monte Carlo rate equation (MCRE) model, the researchers studied the ionization dynamics of argon atoms that received a 480-electronvolt XFEL pulse, in which the resonance-enhanced X-ray multiple ionization mechanism was key to generating otherwise inaccessible charge states. “Based on the computer simulations, we can now understand the very efficient ionization of our samples beyond what was previously believed to be the physical limit,” said Christoph Bostedt, a senior staff scientist at SLAC. “Understanding the process gives you the means to control it.”

XFEL imaging ability relies on the diffract-before-destroy model, in which a high-fluence, ultrashort X-ray pulse generates a diffraction pattern prior to Coulomb explosion; reconstruction of many such patterns will render a 3-D model. Because of the massive number of electronic rearrangements—ranging into the billions and beyond—during the femtosecond X-ray pulse, it is important to gain a better determination of the dynamic response individual atoms have to intense X-ray pulses.

With the optimized MCRE strategy, scientists not only gained the first theoretic verification of resonance-enhanced multiple ionization (REXMI) pathways for inner-shell ionization dynamics of argon atoms, but also verified the REXMI mechanism for earlier observed ultra-efficient ionization in krypton and xenon. The extended MCRE scheme makes possible the hypothetic exploration of resonant high-intensity X-ray physics.

Hard XFEL pluses, such as those available at SLAC’s Linac coherent light source (LCLS) where this experiment was performed, provide extraordinary opportunities to characterize, down to the atomic level, complex systems on ultrafast time scales. Phay Ho and Linda Young of Argonne and Christoph Bostedt and Sebastian Schorb of SLAC developed the extended Monte Carlo rate equation approach.

The study’s findings are slated for publication in the journal Physical Review Letters.

Related Links:
Argonne National Laboratory
SLAC US National Accelerator Laboratory