The Non-Periodic X-ray Imaging Group at PULSE

Imaging coherent dynamics in complex environments with High-energy X-ray Scattering:

The new LCLS capabilities combining high-energy X-rays with extreme brilliance and time resolution can enable us to robustly characterize the non-equilibrium properties of molecules with atomic-scale resolution in time and space, and as such presents a significant opportunity to advance the discovery of design rules for controlling electronic excited states.

We led the first high-energy (18 keV) ultrafast X-ray scattering experiment in solution aiming to disentangle solute-solvent dynamics at XCS, and introduced a single-shot ultra-wide-angle scattering modality that may dramatically improve the scattering vector range. Using a novel inversion approach, we resolve multiple excited and ground state motions in real space and disentangle the solute dynamics from the environment reorganization.  

Imaging complex photodissociation and energy redistribution of a transition metal complex:

The photodissociation dynamics of transition metal carbonyls are crucial for understanding catalytic intermediates, but their ultrafast structural dynamics have been difficult to capture. Using ultrafast X-ray scattering, we obtained the immediate photochemical reactions of Fe(CO)5, observing oscillations in atomic distances and a rotating CO ligand release in the axial direction after photoexcitation. We also observe a secondary CO release and provides detailes into the energy redistribution within the molecule, significantly advancing the understanding of the structural dynamics in transition metal catalysis.

Computational analysis methods development:

Direct real-space recovery of general and complex atomic motions is still mostly limited to signal interpretation in reciprocal space using system-dependent simulations, due to the insufficient scattering vector and photon energies available. We show how to extend super-resolution methods that transformed microscopy and bio-imaging, to the challenging case of ultrafast scattering, where traditional imaging optics, engineered single-emitters, or access to multiple scattering and high spatial frequencies are not available. We introduce theoretically and demonstrate experimentally an inversion and super-resolution method that allows the recovery of multiple sub-diffraction-limit spaced atomic distances from noisy signals. The approach directly brings real-space atomic resolutions to the ultrafast timescale, where often only spectroscopic information is recorded. Read more here... 

Imaging coherently controlled dynamics:

Since the advent of femtochemistry, researchers have endeavored to both manipulate and monitor chemical dynamics at the latent length and timescales of atoms and their bonds. Separately, these goals have been experimentally realized through advances in non-linear laser spectroscopy, and time-resolved scattering measurements of high-energy radiation. In this contribution, we consolidate, for the first time, coherent control and ultrafast X-ray scattering approaches to directly visualize optically steered wavepackets in a benchmark molecular system. Through a Tannor-Kosloff-Rice excitation scheme, we deploy a 520 nm ‘pump’ to photoexcite diatomic iodine vapor from the X state to the B state, and a time-delayed 800 nm ‘dump’ to stimulate population transfer back to the X state. We track the excited charge density at angstrom and femtosecond scales using ultrashort 9 keV X-ray ‘probe’ pulses enabled by the LCLS free electron laser. Legendre decomposition of the high-order anisotropic scattering components allow us to retrieve information on the efficiency of the multi-photon two-color Raman transitions, and their resultant dynamics, at different pump-dump delays. By cross-refencing these observations against numerical solutions of the time-dependent Schrödinger equation, we parameterize the specific Franck-Condon windows that govern light-driven intramolecular dephasing, relaxation and dissociation channels in I2. Finally, we implement Fourier-based inversion of the scattering signals from reciprocal to real space to create spatiotemporal movies of wavepacket evolution along each of these pathways. In addition to providing fresh perspectives on intensely explored photochemical phenomena, the methods herein represent a new class of techniques for directing and detecting quantum dynamics in molecular and condensed media.  

Imaging multiphoton and strong field processes as they evolve: 

Higher orders of anisotropy play a significant role in understanding and probing cases where the molecular system is in the presence of multi-photon absorption and strong laser fields, such as dissociation due to bond softening, above-threshold dissociation, and light-induced conical intersections. In addition, the interaction of ultrashort pulses with molecules with anisotropic polarizability will generate non-adiabatic (or impulsive) alignment. Molecular alignment is often used to probe diverse phenomena in the molecular frame.

We present the first demonstration of ultrafast X-ray scattering of strongly driven molecular Iodine and analysis of high-order anisotropic components of the scattering signal, up to four-photon absorption, and outline a method to analyze the scattering signal using Legendre decomposition. We use simulated anisotropic scattering signals and Fourier analysis to map how anisotropic dissociation motions can be extracted from the various Legendre orders. We observe a multitude of dissociation and vibration motions simultaneously arising from various multiphoton transitions. We use the anisotropy information of the scattering signal to disentangle the different processes and assign their dissociation velocities on the Angstrom and femtosecond scales de-novo.

Image ultrafast structural dynamic at interfaces with atomic resolution:

Studying ultrafast dynamics at solid-liquid interfaces is key to understanding surface reactions and energy transfer in catalysis.  We have created a new program (LDRD) that will enable real-space recovery of atomic density motions of solvents at surfaces and interfaces. The project started in the summer of 2024. Stay tuned!

Imaging excited dynamics in ensembles of molecules in the gas phase:

We have demonstrated in LCLS a molecular movie that resolves atomic motion with time and space resolution of ~30 fs and ~0.3 Å, using time-resolved femtosecond x-ray diffraction patterns from laser-excited molecular iodine. We first excited a gas cell of molecular iodine vapor with a weak ultrafast pulse at 520nm, and subsequently probed the excited ensemble of molecules with an X-ray pulse at 9 keV, with delays of 20 fs. The raw image was then filtered by a Legendre decomposition procedure to produces time-resolved anisotropy maps that are effectively molecular “movies” that filter specific information from different physical process that take place. We can then apply Fourier analysis and deconvolution methods to obtain real-space movies directly. For example, using the β₂ anisotropy information we filters dynamics originating from one-photon transitions of ground X-state to the bound excited B-state as well as the unbound B’ states. Thus, dissociation, represented by short period modulations in the Q-axis, is enhanced on top of the vibration signal (long period Q modulations), which decays due to rotational dephasing in about 1 ps.

References

1. "Real-space Observation of a Transition Metal Complex Dissociation and Energy Redistribution", A. Schori, et al ChemRxiv 10.26434 (2024) 
2. "Characterization of Deformational Isomerization Potential and Interconversion Dynamics with Ultrafast X-ray Solution Scattering", N. E. Powers-Riggs et al, J. Am. Chem. Soc. , 817, 4 (2024)
3. "Real-Space Inversion and Super-Resolution of Ultrafast X-ray Scattering using Natural Scattering Kernels", A Natan,  Physical Review A 107 (2) 023105 (2023)
4. "Transient vibration and product formation of photoexcited CS2 measured by time-resolved x-ray scattering", I Gabalsky et al,  The Journal of Chemical Physics 157 (16) 164305 (2022)
5. "Resolving multiphoton processes with high-order anisotropy ultrafast X-ray scattering", A Natan, A Schori, G Owolabi, J P Cryan, J M Glownia, P H Bucksbaum, Faraday Discussions, 228, 23-138 (2021)
6. "Time-resolved diffraction: general discussion", F Allum et-al, Faraday Discussions 228, 161-190 (2021)
7. "X-ray scattering signatures of early-time accelerations in iodine dissociation", I Gabalski, M R Ware, P H Bucksbaum, Journal of Physics B: Atomic, Molecular and Optical Physics, 53 (24), 244002 (2020)
8. "Imaging Molecular Dynamics of Non-Periodic Systems with Ultrafast X-ray Scattering", A Natan,  Bulletin of the American Physical Society, (2020).
9. "Observation of non-ballistic dissociation trajectories in iodine pump-probe x-ray scattering experiments", I Gabalski, M Ware, P Bucksbaum - Bulletin of the American Physical Society, (2020).
10. “Characterizing multiphoton excitation using time-resolved X-ray scattering”, P H Bucksbaum, M R Ware, A Natan, J P Cryan, J M Glownia,  Physical Review X  10 (1), 011065 (2020).
11. “X-ray diffractive imaging of controlled gas-phase molecules: Toward imaging of dynamics in the molecular frame” T. Kierspel, A. Morgan, J. Wiese, T. Mullins, A. Aquila, A. Barty, R. Bean, R. Boll, S. Boutet, P. Bucksbaum, H. N. Chapman, L. Christensen, A. Fry, M. Hunter, J. E. Koglin, M. Liang, V. Mariani, A. Natan, V. Petrovic, J. Robinson, D. Rolles, A. Rudenko, K. Schnorr, H. Stapelfeldt, S. Stern, J. Thøgersen, C. Hong Yoon, F. Wang, and J. Küpper. Journal of Chemical Physics, 152 (8), 084307 (2020).
12.  “On the limits of observing motion in time-resolved x-ray scattering”. M. R. Ware, J. M. Glownia, A. Natan, J. P. Cryan, and P. H. Bucksbaum. Phil. Trans. R. Soc. A. 377(2145), p.20170477 (2019).
13. "Characterizing dissociative motion in time-resolved x-ray scattering from gas-phase diatomic molecules", Matthew R Ware, James M Glownia, Noor Al-Sayyad, Jordan T O'Neal, Philip H Bucksbaum, Physical Review A 100(3), 033413 (2019)
14. "Fourier-transform inelastic x-ray scattering: A new kind of gas-phase vibrational spectroscopy", Ware M., Glownia J. M., Natan A., Cryan J., and Bucksbaum P. (2018), in Conference on Lasers and Electro-Optics, OSA Terchnical Digest (online) (Optical Society of America, 2018), paper FM4F.5.
15. “Glownia et al. Reply”, JM Glownia, et-al  Physical Review Letters 119 (6), 069302 (2017)
16.  “Simultaneous x-ray imaging of A and B state dynamics in iodine at the LCLS”,  M Ware, A Natan, J Cryan, P Bucksbaum, J Glownia,, Bulletin of the American Physical Society, (2017)
17.  “Filming nuclear dynamics of iodine using x-ray diffraction at the LCLS”, M Ware, A Natan, J Glownia, J Cryan, P Bucksbaum,  Bulletin of the American Physical Society (2017)
18. “Self-referenced coherent diffraction X-ray movie of Ångstrom-and femtosecond-scale atomic motion” JM Glownia, et-al, Physical review letters 117 (15), 153003 (2017)