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NPI: Non-Periodic ultrafast X-ray Imaging

Principal investigator: Adi Natan

Staff and Students: Aviad Schori, Matthew Ware, James 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.

 

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. The outcome of the decomposition 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 on each of the anisotropy coefficients to obtain real-space movies directly. For example, the β2 anisotropy components 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 recently found that higher-order terms capture additional information beyond the number of photons absorbed by the systems. For example, a β4 term appears at a delay of about 55 fs after the  β2 term that preserves both the bound and dissociation information found for β2.   This delayed appearance of a higher order term could be caused by mixing of nuclear wavepackets on two coherently prepared states that couple as they pass through an avoided crossing.  This type of signal is separable using the Legendre decomposition method.  This observation provides a new way to study transitions in more complex excited molecular systems, and a possible approach to identify dynamics near conical intersections, and level crossings, which are ubiquitous in larger systems. We were able to reconstruct a movie of the nuclear probability density arising from this interference.

 

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.

 

Fourier Transform inelastic X-ray scattering for gas phase targets:

We have extended the Fourier-transform inelastic x-ray (FTIX) scattering technique to the gas phase, enabling identification of harmonic, anharmonic, and dissociative motion. This technique was successfully used in the past to obtain dispersion curves for solids to high precision, and 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, and create a superposition of vibrational states on the ground electronic state which after some time delay is probed using x-ray scattering. In this excitation, the ground vibrational state acts as a heterodyne reference for the excited vibrational states, which modulates the scattered x-ray pattern. Using the ground state as a reference amplifies the measured Raman signal by factor of 20 when comparing the excitation fraction to the maximum scattered signal amplitude. The FTIX method is attractive to isolate and understand the details of how vibration and dissociation signals can be obtained without the need to transform in momentum space.

the FT-IXS spectra (radially integrated scattering) at two wavelengths measured across a 3 ps delay range. (A) Exciting with 800 nm, we observe a single beat frequency at the fundamental of the X state around 40.5 THz. (B) Exciting with 520 nm, we observe the dissociation along the QvB’’ dashed line, the slope of which yield the dissociation velocity of  ~17.4 A/ps. In addition, we observe the fundamental vibrational mode and it’s first overtone at 11.4 and 22.8 THz.  

the FT-IXS spectra (radially integrated scattering) at two wavelengths measured across a 3 ps delay range. (A) Exciting with 800 nm, we observe a single beat frequency at the fundamental of the X state around 40.5 THz. (B) Exciting with 520 nm, we observe the dissociation along the QvB’’ dashed line, the slope of which yield the dissociation velocity of  ~17.4 A/ps. In addition, we observe the fundamental vibrational mode and it’s first overtone at 11.4 and 22.8 THz.

 

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.  “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. (accepted)
  2. "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.
  3. "Seeing an electronic transition using ultrafast x-ray diffraction", Natan A., Ware, M., A., Glownia, J., Cryan, & Backsbaum, P. H.,  In preparation.
  4. “Filming non-adiabatic population transfer with x-ray diffraction”, MR Ware, JM Glownia, JP Cryan, R Hartsock, A Natan, PH Bucksbaum, arXiv:1708.03847, Physical review A (under review)
  5. “Glownia et al. Reply”, JM Glownia, et-al  Physical Review Letters 119 (6), 069302 (2017)
  6.  “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)
  7.  “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)
  8. “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)