Fenglin Wang

Noise-robust coherent diffractive imaging with a single diffraction pattern

The resolution of single-shot coherent diffractive imaging at X-ray free-electron laser facilities is limited by the low signal-to-noise level of diffraction data at high scattering angles. The iterative reconstruction methods, which phase a continuous diffraction pattern to produce an image, must be able to extract information from these weak signals to obtain the best quality images. Here we show how to modify iterative reconstruction methods to improve tolerance to noise. The method is demonstrated with the hybrid input-output method on both simulated data and single-shot diffraction patterns taken at the Linac Coherent Light Source.

Limitations of coherent diffractive imaging of single objects due to their damage by intense x-ray radiation

During the coherent diffraction imaging (CDI) of a single object with an intense x-ray free-electron laser (FEL) pulse, the structure of the object changes due to the progressing radiation damage. Electrons are released from atoms and ions during photo-, Auger- and collisional ionization processes. More and more ions appear in the sample. The repulsive force between ions makes them move apart. Form factors of the created ions are reduced when compared with the atomic form factors. Additional scattering of energetic photons from the free electrons confined within the beam focus deteriorates the obtained diffractive signal. Here, we consider pulses short enough to neglect ionic movement and investigate how (i) the decrease of atomic form factors due to the progressing ionization of the sample and (ii) the scattering from the free electrons influence the signal obtained during the CDI. We quantify the loss of structural information about the object due to these effects with hydrodynamic simulations. Our study has implications for the experiments planned on high-resolution three-dimensional imaging of single reproducible particles with x-ray FELs.

Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses

Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angström spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 Å) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.

Femtosecond dark-field imaging with an X-ray free electron laser

The emergence of femtosecond diffractive imaging with X-ray lasers has enabled pioneering structural studies of isolated particles, such as viruses, at nanometer length scales. However, the issue of missing low frequency data significantly limits the potential of X-ray lasers to reveal sub-nanometer details of micrometer-sized samples. We have developed a new technique of dark-field coherent diffractive imaging to simultaneously overcome the missing data issue and enable us to harness the unique contrast mechanisms available in dark-field microscopy. Images of airborne particulate matter (soot) up to two microns in length were obtained using single-shot diffraction patterns obtained at the Linac Coherent Light Source, four times the size of objects previously imaged in similar experiments. This technique opens the door to femtosecond diffractive imaging of a wide range of micrometer-sized materials that exhibit irreproducible complexity down to the nanoscale, including airborne particulate matter, small cells, bacteria and gold-labeled biological samples.

Strongly aligned gas-phase molecules at free-electron lasers.

Here, we demonstrate a novel experimental implementation to strongly align molecules at full repetition rates of free-electron lasers. We utilized the available in-house laser system at the coherent x-ray imaging beamline at the linac coherent light source. Chirped laser pulses, i.e., the direct output from the regenerative amplifier of the Ti:Sa chirped pulse amplification laser system, were used to strongly align 2, 5-diiodothiophene molecules in a molecular beam. The alignment laser pulses had pulse energies of a few mJ and a pulse duration of 94 ps. A degree of alignment of

Automated identification and classification of single particle serial femtosecond X-ray diffraction data

\langle {\mathrm{cos}}^{2}{\theta }_{2{\rm{D}}}\rangle =0.85

Conformation sequence recovery of a non- periodic object from a diffraction-before- destruction experiment

was measured, limited by the intrinsic temperature of the molecular beam rather than by the available laser system. With the general availability of synchronized chirped-pulse-amplified near-infrared laser systems at short-wavelength laser facilities, our approach allows for the universal preparation of molecules tightly fixed in space for experiments with x-ray pulses.

Characterizing the focus of a multilayer coated off-axis parabola for FLASH beam at λ = 4.3 nm

Kinetic Boltzmann equations are used to describe electron emission spectra obtained after irradiation of noble-gas clusters with intense vacuum ultraviolet (VUV) radiation from a free-electron-laser (FEL). The experimental photoelectron spectra give a complementary and more detailed view of nonlinear processes within atoms and clusters in an intense laser field compared to mass spectroscopy data. Results from our model obtained in this study confirm the experimental and theoretical findings on the differing ionization scenarios at longer (100?nm) and shorter (32?nm) VUV radiation wavelengths. At the wavelength of 100?nm the thermoelectronic electron emission dominates the emission spectra. This indicates the plasma formation and the inverse bremsstrahlung (IB) heating of electrons inside the plasma. This effect is clearly visible for xenon (with the fitted temperature of 6?7?eV), and less visible for argon (with the fitted temperature of 2?3?eV). The two-photon-ionization rate for argon that initiates the cluster ionization, is much lower than the single-photoionization rate for xenon. Also, more of the photoelectrons created within an argon cluster are able to leave it, as they are more energetic than those released from a xenon cluster. Therefore, the IB heating of plasma electrons in argon is less efficient than in xenon, as the density of the electrons remaining within the cluster is lower.At a wavelength of 32?nm the dominant ionization mechanism identified from the electron spectra of argon clusters is the direct multistep ionization. The signature of the thermalization of electrons is also observed. However, as the heating of electrons due to the inverse bremsstrahlung process is weak at these radiation wavelengths and pulse fluences, the increase of the electron temperature with the pulse intensity is mainly due to the increasing photoionization rate within the irradiated sample.