Session C5: Focus Session: Time-resolved Electron Diffraction and Novel Electron Beam Sources

Ultrafast Imaging of Molecules with Electron Diffraction

Ultrafast imaging of isolated molecules in three dimensions and with atomic resolution is important for elucidating intermediate states in molecular reactions. Electron diffraction has been the main tool to determine the structure of molecules in the gas phase. Diffraction patterns from randomly oriented molecules in the gas phase contain only one-dimensional information, and thus input from theoretical models is needed to recover the structure. We have shown experimentally that three-dimensional structural information of symmetric top molecules can be retrieved from multiple electron diffraction patterns of aligned molecules. The molecules are aligned impulsively with a femtosecond laser pulse and probed with a femtosecond electron pulse two picoseconds later, when the degree of alignment reaches a maximum. Furthermore, we show that our method can be extended to asymmetric molecules using diffraction from partially aligned molecules and a new structure retrieval method. Previous reconstruction algorithms had two major limitations: First, they require diffracting from a single molecule or an ensemble with very high degree of alignment that is generally incompatible with diffraction experiments, and second, each algorithm is only applicable for a specific type of molecules. We have developed a two-step reconstruction comprising a genetic algorithm that corrects for the imperfect alignment followed by an iterative phase retrieval method in cylindrical coordinates. Our simulations show that the full 3-D structure of trifluorotoluene, an asymmetric-top molecule, can be reconstructed with atomic resolution.   

Imaging Coherent Electronic Motion in Atoms by Ultrafast Electron Diffraction

Ultrafast electron diffraction from time-varying coherent electronic states of the H atom is investigated.\footnote{H.-C.~Shao and A.F.~Starace, Phys.~Rev.~A \textbf{88}, 062711 (2013).} Electron diffraction from coherent electronic states exhibiting breathing and wiggling modes of electronic motion are simulated in order to demonstrate the capability of attosecond electron pulses to image electron dynamics. A theoretical analysis identifies the conditions necessary to obtain time-resolved measurements. The scattering patterns and their temporal behaviors are shown to differentiate the two kinds of target electronic motion. Moreover, our simulations show that inelastic processes contribute significantly to the diffraction patterns. Thus although the diffraction patterns clearly distinguish different modes of target electronic motion, they cannot be easily related to the time-dependent target charge density. Finally, we note that detection of the scattered electron energy can provide more information on time-dependent target electronic motion.   

Femtosecond few-hundreds-of-keV electron pulses from direct laser acceleration in a low-density gas

Subrelativistic electrons are a valuable tool for high-resolution atomic and molecular imaging. In particular, electron pulses with energies ranging from 50 to 300 keV have been successfully used in time-resolved ultrafast electron diffraction (UED) experiments to probe physical phenomena on a subpicosecond time scale. Laser-driven electron acceleration has been proposed as an alternative to the static accelerator technology currently in use. In principle, it has several advantages: (i) the short wavelength of the accelerating field may lead to electron bunches with duration of the order of 10 fs or less; (ii) there is an intrinsic synchronization between the electron probe and the laser pump; and (iii) using a gas medium, the electron source is self-regenerating and could be used for UED experiments at high repetition rates. Using three-dimensional particle-in-cell simulations, we showed that 240-keV electron pulses with 1-fs initial duration and 5\% energy spread could be produced by radially polarized laser pulses focused in a low-density hydrogen gas [Marceau, et al., Phys. Rev. Lett. 111, 224801 (2013)]. The latest results suggest that 100-500 keV energy with similar duration is within reach of the actual laser technology.   

Mapping molecular motions leading to charge delocalization with ultrabright electrons

Ultrafast diffraction has broken the barrier to atomic exploration by combining the atomic spatial resolution of diffraction techniques with the temporal resolution of ultrafast spectroscopy. X-ray free electron lasers, slicing techniques and femtosecond laser-driven X-ray and electron sources have been successfully applied for the study of ultrafast structural dynamics in a variety of samples. Yet, the application of fs-diffraction to the study of rather sensitive organic molecular crystals remains unexplored. Organic crystals are composed by weak scattering centres, often present low melting points, poor heat conductivity and are, typically, radiation sensitive. Low repetition rates (about tens of Hertz) are therefore required to overcome accumulative heating effects from the laser excitation that can degrade the sample and mask the structural dynamics. This imparts tremendous constraints on source brightness to acquire enough diffraction data before adverse photo-degradation effects have played a non-negligible role in the crystalline structure. We implemented ultra-bright femtosecond electron diffraction to obtain a movie of the relevant molecular motions driving the photo-induced insulator-to-metal phase transition in the organic charge-transfer salt (EDO-TTF)$_{\mathrm{2}}$PF$_{\mathrm{6}}$. On the first few picoseconds (0 - 10 ps) the structural evolution, well-described by three main reaction coordinates, reaches a transient intermediate state (TIS). Model structural refinement calculations indicate that fast sliding of flat EDO-TTF molecules with consecutive motion of PF$_{\mathrm{6}}$ counter-ions drive the formation of TS instead of the expected flattening of initially bent EDO-TTF moieties which seems to evolve through a slower thermal pathway that brings the system into a final high temperature-type state. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology. For more information vide-infra Gao et al., Nature 496, 343 (2013) and references there in.   

Intense Laser Acceleration of Electrons in Highly-Charged Ions to GeV Energies

Recent advances in laser technology have led to the development of short-pulse high-power petawatt lasers, making possible laser intensities of the order of $\mathrm{10^{22}~W/cm^2}$. This development opens the highly relativistic regime of light-matter interactions, raising interest in both practical and fundamental studies. One of the novel phenomena is the acceleration of electrons to GeV energies.\footnote{S. X. Hu and A. F. Starace, Phys. Rev. Lett. \textbf{88}, 245003 (2002).}$^,$\footnote{S. X. Hu and A. F. Starace, Phys. Rev. E \textbf{73}, 066502 (2006).} An electron in a highly-charged ion can be ionized in a laser field at its peak intensity and violently accelerated to nearly the speed of light, then surf on the laser field to GeV energy. We use the Classical Trajectory Monte Carlo (CTMC) method to simulate the intense laser acceleration of electrons in highly-charged ions. For tightly-focused laser fields, we take into account up to fifth order corrections to the paraxial approximation, which lead to longitudinal laser field components in the focus region. We report here our recent finding that \emph{the final-state energies and ejection angles of the electrons depend on the initial target ion positions relative to the laser focus}.