NPI: Non-Periodic Ultrafast X-ray Imaging

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.

In March 2021 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 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.

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 complex photodissociation of transition metal complex:

An understanding at the atomic level of how transition-metal complexes catalyze reactions, and in particular, the role of the short-lived and reactive intermediate states involved is of great importance for future control of photocatalytic hydrogen production and selective carbon–hydrogen bond activation. The photo-physics governing the formation of intermediate complexes such as Fe(CO)4 has received a lot of attention, often focusing on the reaction pathways and molecular structures of these transient species. We have evidence that the metal-to-ligand charge-transfer (MLCT) transition creates a coherent vibration in the trigonal bipyramidal complex that in turn induces a synchronized sequential dissociation of the first CO ligand. We also observe a second thermal CO loss with a striking different rate where the ligand is restricted in real space near the parent molecule.

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.

Imaging coherently controlled dynamics:

One of the most successful coherent control approaches in the time domain is the Tannor-Rice pump-dump scheme. In this scheme we steer an excited wavepacket into a specific state by coinciding a delayed dump pulse with a proper timed evolution of wavepacket that was born when a pump pulse excites the molecule into a particular Franck Condon region. Experimental demonstrations of this control has been performed by several groups via non-linear spectroscopy. Here, we offer a direct imaging approach of such a punp-dump experiment where we probe the excited charge density of a pump-dump scheme with a delayed X-ray probe pulse. As a result, we have access to the entire dynamics including additional states that participate as a result of time reversed dump-pump sequences. We used 520 nm to pump population from the X to the B state in diatomic Iodine vapor, and a delayed 800nm pulse to dump the population back to the X state at a larger internuclear separation. We used the Legendre decomposition approach that have developed to filter the dumped population and saw how the control scheme was effective at the proper timing of the wavepacket evolution near the outer turning point of the B state.  We have also mapped a secondary channel where the 800nm dump pulse acts as a pump pulse from the ground X-state to the A-state.  We further developed a Fourier decomposition in the temporal delay domain to retrieve physical parameters and efficiency of such processes as well as dissociation.

Delay frequency-resolved x-ray scattering:

We can also determine motions by utilizing a method similar to the Fourier-transform inelastic x-ray scattering technique, which has been effectively used before to achieve precise dispersion curves for solids. We demonstrate the ability to measure the dissociation velocity and vibrational excitation on molecular iodine with high precision. For example, we induce a resonant Raman transition in molecular iodine using strong 800nm pulses. This method is an independent way to separate bound from dissociative motion. This is because bound motion, which is mostly manifested by constant vibrational frequencies appears as stationary peaks in the frequency domain, while dissociative motion appears as straight lines along ω_k=v_k Q, with slopes v_k as the effective dissociation velocities. We derive this relation and use FRXS to extract state-specific dynamics from experimental scattering patterns from molecular iodine. We use this method to also resolve dissociation via one- and two-photon absorption as well as vibrational wave packets.

References

1. "Real-Space Inversion and Super-Resolution of Ultrafast X-ray Scattering using Natural Scattering Kernels", A Natan,  Physical Review A 107 (2) 023105 (2023)
2. "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)
    
3. "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)
4. "Time-resolved diffraction: general discussion", F Allum et-al, Faraday Discussions 228, 161-190 (2021)
5. "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)
6. "Imaging Molecular Dynamics of Non-Periodic Systems with Ultrafast X-ray Scattering", A Natan,  Bulletin of the American Physical Society, (2020).
7. "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).
8. “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).
9. “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).
10.  “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).
11. "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)
12. "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.
13. “Glownia et al. Reply”, JM Glownia, et-al  Physical Review Letters 119 (6), 069302 (2017)
14.  “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)
15.  “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)
16. “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)