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NPI: Non-Periodic ultrafast X-ray Imaging

Principal investigator: Adi Natan

Postdocs and Students: Aviad Schori, Ian Gabalsky, Brandon Zhang, Grace Owolabi

Collaborators : Phil Bucksbaum, Kelly Gaffney, Elisa Biasin, James Cryan, Thomas Wolf, Tim van-Driel (LCLS), Hasan Yavas(LCLS), Mike Glownia (LCLS)

 

 

Project scope: The NPI program studies the nature of time-resolved short wavelength scattering on photoexcited molecular systems. We explore ways to image quantum dynamics de-novo using experimental and computational approaches with the aim to produce molecular movies of structural dynamics of systems of increased complexity in the sub-angstrom and femtosecond scales. We have implemented signal decomposition analysis to image different physical mechanism of excited systems such as electronic population transfer, vibrational motion, dissociation, rotational dephasing and Raman transitions in a diatomic system. We are extending or efforts to polyatomic systems and dynamics of a driven systems in a solvent environment.  We help  develop effective science protocols for x-ray diffractive scattering experiments, demonstrate important new capabilities as soon as they become feasible at LCLS and leverage development of new modalities and detection schemes.

 

Research Interests:

 

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 improved the scattering vector range up to 14 A-1 . 

Preliminary time-resolved signal difference of PtPOP  in ACN (solvent subtracted)

 

Imaging multiphoton and strong field processes as they evolve using High-order anisotropy scattering:

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:

Directly resolving in real-space multiple atomic motions using ultrafast x-ray scattering is generally limited by the finite detector range. As a result, signal interpretation mostly relies on modeling and simulations of specific excitation pathways. We developed an approach to resolve ultrafast diffuse x-ray scattering signals in real space and recover multiple atomic motions de-novo. We introduce a scattering basis representation - Natrual Scattering Kernels (NSK) -  that is composed of the measurement parameters and constraints and the subsequent inversion analysis. We then leverage signal priors, such as smoothness and sparsity to deconvolve and super-resolve the spatially transformed signals using convex optimization. We validate the approach on simulated data with detection limits similar to X-ray free-electron laser experiments and find the resolution limits and noise dependence on the accuracy of the recovery.

(a) 1-D radial TDSE simulation of the time-dependent charge density difference for Iodine excited from its ground state by an 520 nm , 30 fs, resulting in vibration and dissociation. (b) The simulated isotropic difference scattering signal assuming 20dB SNR, detector q-range  truncation  0.5<q<4 A-1, discretization, and resampling . (c) Applying a spherical Bessel transform to the truncated scattering signal yields the pair-distribution difference signal. The limited q-range hinders the ability to accurately recover the positions and motions of the calculated charge density. (d)  The NSK dictionary following the measurement constraints is used to deconvolve the pair-distribution difference using (e)  L2 and (f) L1 regularizations.

 

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.

(Left) Legendre (β2) decomposition based analysis of the scattering signal vs. x-ray probe time delay uncovers photoexcited Iodine dynamics. (Right) The real space reconstruction of shows the a) onset of the excited pulse, the B-state is directly over the X-state centered on 2.7 Å. b) vibrational oscillations. c) dissociation d) wavepacket dispersion e) rotational dephasing.

 

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 resolve motions using an analogue of the Fourier-transform inelastic x-ray scattering technique that was successfully used in the past to obtain dispersion curves for solids to high precision. Here, we apply it to analyze the anharmonic vibrations and dissociations of molecular iodine that was measured in our previous work. This approach allows us to obtain a dispersion plot for the system under study and facilitates the retrieval of physical observables without the need to invert the scattering data. We demonstrate the ability to measure the dissociation velocity and vibrational excitation on molecular iodine in high precision. For example, we induce a resonant Raman transition in molecular iodine using strong 800nm pulses. This method is an insependat  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. Moreover, we show how the time onset of dissociation as well as secondary processes such as impulsive Raman scattering, and spontaneous hyper Raman scattering can be resolved.

  

The power spectrum of angle integrated FRXS revealing (a) impulsive Raman and (b) spontaneous hyper-Raman scattering, as well as (c,d) dissociation.

 

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 are led an LCLS beam time aiming to capture a molecular movie of the photo-physics of CO loss from the transition metal carbonyl complex Fe(CO)5 as well as further dissociations to the intermediate complex. We excited Fe(CO)5 with an ultrashort  UV pulse (266nm) and probed with a delayed hard X-ray pulse.  The ultrafast dynamics of such excitation is complex, involves multiple states and is extremely hard to calculate using the state-of-the-art tools. We set to understand to what level the short-term coherence and anisotropy play a role in the onset of the CO loss mechanism of such system.

 

  1. "Real-Space Inversion and Super-Resolution of Ultrafast X-ray Scattering using Natural Scattering Kernels", A Natan, arXiv:2107.05576 (2021)
  2. "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)
  3. "Time-resolved diffraction: general discussion", F Allum et-al, Faraday Discussions 228, 161-190 (2021)
  4. "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)
  5. "Imaging Molecular Dynamics of Non-Periodic Systems with Ultrafast X-ray Scattering", A Natan,  Bulletin of the American Physical Society, (2020).
  6. "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).
  7. “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).
  8. “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).
  9.  “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).
  10. "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)
  11. "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.
  12. “Glownia et al. Reply”, JM Glownia, et-al  Physical Review Letters 119 (6), 069302 (2017)
  13.  “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)
  14.  “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)
  15. “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)