Florian Burmeister

Femtosecond diffractive imaging with a soft-X-ray free-electron laser

Theory predicts1,2,3,4 that, with an ultrashort and extremely bright coherent X-ray pulse, a single diffraction pattern may be recorded from a large macromolecule, a virus or a cell before the sample explodes and turns into a plasma. Here we report the first experimental demonstration of this principle using the FLASH soft-X-ray free-electron laser. An intense 25 fs, 4×1013 W cm−2 pulse, containing 1012 photons at 32 nm wavelength, produced a coherent diffraction pattern from a nanostructured non-periodic object, before destroying it at 60,000 K. A novel X-ray camera assured single-photon detection sensitivity by filtering out parasitic scattering and plasma radiation. The reconstructed image, obtained directly from the coherent pattern by phase retrieval through oversampling5,6,7,8,9, shows no measurable damage, and is reconstructed at the diffraction-limited resolution. A three-dimensional data set may be assembled from such images when copies of a reproducible sample are exposed to the beam one by one10.

Development of a four-element conical electron lens dedicated to high resolution Auger electron–ion(s) coincidence experiments

A four-element conical electron lens has been developed in view of its integration to a double toroidal electron energy analyzer (DTA) dedicated to Auger electron–ion coincidence measurements. The lens design, using electron trajectory numerical simulations, was entirely guided by the perspective of analyzing energetic electrons with high resolution in the multicoincidence regime. The design, construction, and experimental characterization stages of this electron optics system are described in this article. Emphasis is put on the importance of third generation synchrotron radiation sources when performing such multicoincidence experiments.

The dynamic Auger-Doppler effect in HF and DF: control of fragment velocities in femtosecond dissociation through photon energy detuning

Abstract The Auger–Doppler effect in the experimental spectra of HF and DF is presented, and the dynamics of ultra-fast dissociation in the core-excited state are discussed. The Doppler splitting of the atomic Auger peak is calculated and simulated using a classical model and a very good agreement is found between experiment and simulation. It is shown that the difference in photon energy relative to the resonance is transferred completely into the kinetic energy release (KER). This is expected to be a general phenomenon, but is clearly illuminated in the HF/DF case. Thus the fragment velocity can be controlled through photon energy detuning.

Experimental study of photoionization of ozone in the 12 to 21 eV region

The total and partial ion yield of ozone using time-of-flight is presented. The measurements were done using multicoincidence between a photoelectron and a photoion (PEPICO). Comparison with the photoelectron spectrum and previous measurements using other techniques allowed the assignment of most broad features in the spectra. Kinetic energy released is obtained for O+ and O2+ ions. A discussion about the dissociation channels is included.

Is there interference in the resonant Auger electron spectra of N 1s and O 1s -> 2 Pi core excited NO?

High-resolution, angle-resolved resonant Auger electron spectra of the NO molecule in the regions of both N and O 1s→2π core electron excitations are presented. A large number of vibrational final states are resolved due to high energy resolution. Calculations based on lifetime vibrational interference (LVI) theory neglecting interference between different electronic intermediate states and between direct and resonant channels have been performed. A comparison between theoretical and experimental spectra shows that LVI theory describes the major spectroscopic features quite well. The same holds for the evolution of the angular averaged partial cross sections with the change of excitation energy. The angular distribution of particular vibrational final states are, however, not described successfully with LVI calculations at the present level of sophistication. A theoretical analysis supports that one reason for this deviation is electronic state interference.

Molecular alignment of ammonia studied by electron-ion-ion coincidence spectroscopy

Electron-ion-ion coincidence measurements carried out at discrete resonances near the N 1s threshold in ammonia are reported. The measured coincidence spectra show clear alignment of the molecule upon resonant core-electron excitation. The coincidence data are analyzed to extract information about the molecule in the excited state by simulating the alignment and the dissociation processes. Dynamic changes in molecular geometry are found as the photon energy is scanned through the N 1s-->4a(1) resonance, whereas for the N 1s-->2e state the geometry and kinetic energy released upon dissociation remain unchanged. The alignment of the core-excited molecules is found to be preserved even in two-step dissociation processes.

A vibrationally resolved experimental study of the sulfur L-shell photoelectron spectrum of the CS2 molecule

The sulfur L-shell photoelectron spectrum of the carbon disulfide molecule has been studied using monochromated synchrotron radiation with a photon energy of 250 eV. The spectrum is atomic like, showing three major bands that can be associated with the sulfur (2p-1)2P3/2,12 and (2s-1)2S1/2 ionic states. A closer inspection shows that the 2P3/2 state is further split into two components separated by 128 meV due to the molecular field. The resulting ionic states are located at 169.806 eV (0-0 energy), 169.934 eV (0-0 energy), 171.075 eV (0-0 energy) and 237.05±0.2 eV (centroid), respectively. Vibrational progressions in the (2p-1) bands are attributed to the asymmetric ν3 mode, which gives evidence of a localization of the core hole. The following values were obtained for the vibrational constants: ωe = 196.7±1.1 meV; ωexe = 0.2±0.5 meV. A curve fit of the vibrational lines using a Voigt function gave a natural width of 59.6±1.8 meV for the (2p-1) states, corresponding to a lifetime of 11 fs, and a spectrometer broadening of 38.2±1.8 meV. The (2p-1) bands are accompanied by shake-up structures occurring at 6-18 eV higher energies. They are interpreted mainly in terms of excitations to the unoccupied 3πu* orbital in the final ionic state. The (2s-1)2S1/2 band is broad and structureless due to fast Coster-Kronig processes. A fitting of a Voigt function gives a natural line-width of 1.85 eV which corresponds to a lifetime of 0.4 fs.

Polarization dependent effects in photo-fragmentation dynamics of free molecules

We present multicoincidence spectra of nitrogen, formic acid and methyl methacrylate. We demonstrate how to probe the local symmetry of molecular orbitals from molecules core excited with linearly polarized synchrotron radiation. The intensity distribution of the photoelectron photo-ion photo-ion coincidence (PEPIPICO) spectrum reflects the selectivity and localization of core excitation by polarized light. By simulating the spectra the angular dependence of the fragmentation is determined.

Filtering core excitation spectra: vibrationally resolved constant ionic state studies of N 1s → 2π core-excited NO

High-resolution electron spectroscopy studies of the NO molecule in the regions of the N 1s→2π core excitations have been performed. By selecting electrons within certain binding energy ranges, either the Auger electron yield - a good approximation for the x-ray absorption spectrum - or the electrons emitted after decay to a particular ionic final state (constant ionic state (CIS)) were detected. By selecting the X 1Σ+ (2π0) final state, the superposition of several intermediate states can be disentangled by exploiting a selection rule which permits only two of the three dipole-allowed intermediate states to decay to this specific final state. This makes it possible to obtain more detailed information on the potential energy curves of the intermediate states than is available from regular absorption measurements. We have also obtained CIS spectra for individual vibrational sublevels within this state. The role of lifetime vibrational interference on the appearance of these spectra is discussed.

Core localization and sigma(*) delocalization in the O 1s core-excited sulfur dioxide molecule

Electron-ion-ion coincidence measurements of sulfur dioxide at discrete resonances near the O 1s ionization edge are reported. The spectra are analyzed using a model based upon molecular symmetry and on the geometry of the molecule. We find clear evidence for molecular alignment that can be ascribed to symmetry properties of the ground and core-excited states. Configuration interaction (CI) calculations indicate geometry changes in accord with the measured spectra. For the SO(2) molecule, however, we find that the localized core hole does not produce measurable evidence for valence localization, since the transition dipole moment is not parallel to a breaking sigma* O-S bond, in contrast to the case of ozone. The dissociation behavior based upon the CI calculations using symmetry-broken orbitals while fixing a localized core-hole site is found to be nearly equivalent to that using symmetry-adapted orbitals. This implies that the core-localization effect is not strong enough to localize the sigma* valence orbital.