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NPI: Non-Periodic ultrafast X-ray Imaging

Principal investigator: Adi Natan

Postdocs and Students: Aviad Schori, Ian Gabalsky

Collaborators : Phil Bucksbaum, Kelly Gaffney, Elisa Biasin, James Cryan, Matt Ware, Thomas Wolf, 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 diatomic systems in a solvent environment.  We helpdevelop 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 excited dynamics in ensembles of molecules in the gas phase:

We have recently 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.

We have are studying x-ray scattering of a strongly driven molecular systems and recover high-order anisotropy components of the scattering signal, above the single photon absorption limit. The analysis of these anisotropy components allows to resolve and disentangle the complex excitation dynamics that involve multiple electronic states with higher rovibration modes, strong field interaction and multiple dissociation pathways that happen simultaneously.

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.

 

Imaging ultrafast dynamics of simple systems in solvents:

  In June 2017 and August 2018, we studied at SACLA coherent diffraction from diatomic Iodine molecules in different solvents (ethanol and cyclohexane) in order to image molecular motion in the condensed phase using wide angle x-ray scattering. We have also developed molecular dynamics simulations with Rob Parrish to investigate the role of the solvent cage on the photo absorption process and the following dynamics, and how anisotropy can be used to study these dynamics.  We show that under the experimental conditions used, iodine dissociates ballistically between 2.6 and 5 Ang in the first 150 fs of the interaction. Then it rapidly slows down and relaxes into a new bond distance where vibration motion is completely damped. We are in the process of analyzing the data form the experiment as well as modeling the angle resolved cage dynamics to obtain an effective image of iodine in methanol, and the mechanical propertied of the solvent solute interaction.

 

  1. "Imaging Molecular Dynamics of Non-Periodic Systems with Ultrafast X-ray Scattering", A Natan,  Bulletin of the American Physical Society, (2020).
  2. "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).
  3. “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).
  4. “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).
  5.  “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).
  6. "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)
  7. "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.
  8. “Glownia et al. Reply”, JM Glownia, et-al  Physical Review Letters 119 (6), 069302 (2017)
  9.  “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)
  10.  “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)
  11. “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)