Showing papers by "John D. Bozek published in 2017"
TL;DR: Bernando et al. as discussed by the authors used coherent diffractive imaging using an x-ray free electron laser to study the formation of strongly deformed droplets, which is evidenced by large anisotropies and intensity anomalies in the obtained diffraction images.
Abstract: Author(s): Bernando, C; Tanyag, RMP; Jones, C; Bacellar, C; Bucher, M; Ferguson, KR; Rupp, D; Ziemkiewicz, MP; Gomez, LF; Chatterley, AS; Gorkhover, T; Muller, M; Bozek, J; Carron, S; Kwok, J; Butler, SL; Moller, T; Bostedt, C; Gessner, O; Vilesov, AF | Abstract: Rotating superfluid He droplets of approximately 1 μm in diameter were obtained in a free nozzle beam expansion of liquid He in vacuum and were studied by single-shot coherent diffractive imaging using an x-ray free electron laser. The formation of strongly deformed droplets is evidenced by large anisotropies and intensity anomalies (streaks) in the obtained diffraction images. The analysis of the images shows that in addition to previously described axially symmetric oblate shapes, some droplets exhibit prolate shapes. Forward modeling of the diffraction images indicates that the shapes of rotating superfluid droplets are very similar to their classical counterparts, giving direct access to the droplet angular momenta and angular velocities. The analyses of the radial intensity distribution and appearance statistics of the anisotropic images confirm the existence of oblate metastable superfluid droplets with large angular momenta beyond the classical bifurcation threshold.
35 citations
TL;DR: In this article, a femtosecond time-resolved Auger electron spectroscopy study concerning the photofragmentation dynamics of thymine was performed, where the appearance of clearly distinguishable signatures from thymine′s neutral photorefragment isocyanic acid was observed.
Abstract: Molecules often fragment after photoionization in the gas phase. Usually, this process can only be investigated spectroscopically as long as there exists electron correlation between the photofragments. Important parameters, like their kinetic energy after separation, cannot be investigated. We are reporting on a femtosecond time-resolved Auger electron spectroscopy study concerning the photofragmentation dynamics of thymine. We observe the appearance of clearly distinguishable signatures from thymine′s neutral photofragment isocyanic acid. Furthermore, we observe a time-dependent shift of its spectrum, which we can attribute to the influence of the charged fragment on the Auger electron. This allows us to map our time-dependent dataset onto the fragmentation coordinate. The time dependence of the shift supports efficient transformation of the excess energy gained from photoionization into kinetic energy of the fragments. Our method is broadly applicable to the investigation of photofragmentation processes.
19 citations
TL;DR: In this article, the Resonant Auger (RA) decay spectra of carbon molecules were recorded with angular resolution using linearly polarized synchrotron radiation and were interpreted with the help of ab initio electronic structure and dynamics calculations, which account for electronic-state interference between the direct and different resonant ionization pathways.
Abstract: Resonant Auger (RA) decay spectra of carbon $1s$ excited ${\mathrm{CH}}_{3}\mathrm{Cl}$ molecules are recorded with angular resolution using linearly polarized synchrotron radiation. The selected photon energies corresponding to the C $1s\ensuremath{\rightarrow}8{a}_{1}$ core to lowest unoccupied molecular orbital and C $1s\ensuremath{\rightarrow}4s{a}_{1}$, $4pe$, and $4p{a}_{1}$ core to Rydberg excitations of methyl chloride are used and electrons in the binding energy range of 11--37 eV are detected. The vibrationally unresolved RA electron angular distributions, recorded for participator Auger transitions populating the $X$, $A$, $B$, and $C$ states of the ${\mathrm{CH}}_{3}{\mathrm{Cl}}^{+}$ ion, exhibit strong variations across the selected electronic resonances. These observations are interpreted with the help of ab initio electronic structure and dynamics calculations, which account for electronic-state interference between the direct and different resonant ionization pathways. For spectator transitions, the theory predicts almost isotropic angular distributions with moderate changes of $\ensuremath{\beta}$ parameters around zero, which is in agreement with the experimental observations.
9 citations
TL;DR: In this article, the relative cross sections for the valence shell photoionisation (PI) of ground level and metastable Ca$+} ions were measured with high energy resolution by using the ion-photon merged-beams technique at the Advanced Light Source.
Abstract: Relative cross sections for the valence shell photoionisation (PI) of $\rm ^2S$ ground level and $\rm ^2D$ metastable Ca$^{+}$ ions were measured with high energy resolution by using the ion-photon merged-beams technique at the Advanced Light Source. Overview measurements were performed with a full width at half maximum bandpass of $\Delta E =17$~meV, covering the energy range 20~eV -- 56~eV. Details of the PI spectrum were investigated at energy resolutions reaching the level of $\Delta E=3.3$~meV. The photon energy scale was calibrated with an uncertainty of $\pm5$~meV. By comparison with previous absolute measurements %by Kjeldsen et al in the energy range 28~eV -- 30.5~eV and by Lyon et al in the energy range 28~eV -- 43~eV the present experimental high-resolution data were normalised to an absolute cross-section scale and the fraction of metastable Ca$^{+}$ ions that were present in the parent ion beam was determined to be 18$\pm$4\%. Large-scale R-matrix calculations using the Dirac Coulomb approximation and employing 594 levels in the close-coupling expansion were performed for the Ca$^{+}(3s^23p^64s~^2\textrm{S}_{1/2})$ and Ca$^{+}(3s^2 3p^6 3d~^2\textrm{D}_{3/2,5/2})$ levels. The experimental data are compared with the results of these calculations and previous theoretical and experimental studies.
4 citations
2 citations
SLAC National Accelerator Laboratory1, Max Planck Society2, Argonne National Laboratory3, Dublin City University4, Ohio State University5, Ludwig Maximilian University of Munich6, European XFEL7, Korea Research Institute of Standards and Science8, University of Hamburg9, National Science Foundation10
TL;DR: In this paper, a single-shot X-ray pulse pair is generated and characterized using photoelectron streaking spectroscopy, and the femtosecond resolution is obtained.
Abstract: Arbitrary manipulation of the temporal and spectral properties of X-ray pulses at free-electron lasers (FELs) would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the FEL driving electron bunch can be tuned to emit a pair of X-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the X-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the X-rays is essential to ensure precise and consistent control. In this work, we have generated X-ray pulse pairs and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future X-ray pulse shaping techniques.