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Controlling sample integrity in FEL experiments: Exposure, heating, and plasma dynamics in the time domain

Lupe

Free-electron lasers offer the possibility of single-shot imaging of free particles in the gas phase. Due to the high power density of the FEL radiation needed for high resolution Coherent Diffractive Imaging, the sample will be turned into a highly excited plasma following exposure. This can lead to the loss of image contrast as the objects structure is changing during the illumination.

The four-year research program, which is jointly funded by the BMBF (Germany) and the Swedish Research Council, will support the collaboration with our partners Jakob Andreasson and Nicusor Timneanu from the Laboratory of Molecular Biophysics at Uppsala University in Sweden within the Röntgen-Ångström-Cluster.

Our aim is to refine the techniques for sample delivery that allow handling of inorganic, organic and biological nanostructures, and to develop time-resolved spectroscopy methods to follow the dynamics of nanometer to micrometer sized plasmas on ultra-short  time scales.

Real time mapping of XUV-induced cluster charging using THz streaking (current project)

Lupe

The idea of an atomic transient recorder which stems from the attosecond metrology (see for example R.Kienberger et al., Nature 427 (2004) 817-821) is a promising technique to access the dynamics within the exciting pulse itself. In this approach the electrons created upon photoionization with an attosecond soft-X-ray pulse from a high harmonic generation source (HHG) are accelerated or decelerated by the electrical field of the near-infrared (NIR) laser. By changing the time-delay between soft-X-ray and NIR pulses, i.e. by varying the phase of the NIR streaking field with respect to the electron photoemission time, and measuring the electron kinetic energy spectra a streaking spectrogram is obtained. From this spectrogram the temporal structure of the electron wave packet can be reconstructed.

Using this approach the cascaded Auger decay process upon the ionization of the Kr 3d subshell has been resolved at the attosecond HHG source of TU Vienna (A.J. Verhoef et al., New J. Phys. 2011). For the analysis of the temporal structure of the electron wave packets produced by FEL pulses with duration of 10-100 fs a longer electrical field oscillation period is required. This has been realized by using a long-wavelength (>100 μm) in the terahertz-field-driven X-ray streak camera (U. Frühling et al., Nature Photonics 2009) at the THz beamline at FLASH. Recently we have performed the first streaking experiments on Xe clusters following ionization with intense 92 eV FEL pulses using THz radiation from the undulator. The idea of the experiment is to reveal time scales for direct electron photoemission and dynamics of nanoplasma formation inside the cluster. Key questions are related to a detailed understanding of the way that high intensity FEL pulses interact with the electronic structure of nano-samples.   

Watching a molecular explosion

Molecular explosion - X-ray laser FLASH probes ultra-fast electron rearrangement
Lupe

Intense light pulses can produce highly excited nonequilibrium states of matter within femtoseconds (fs). Simple diatomic molecule like iodine exposed to the strong near-infrared (NIR) laser field will explode into energetic atomic fragments. The dynamics of molecular explosion proceeds through free essential steps – creation of molecular ion, valence electron localization and charge separation. The experimental challenge is to track the dynamical details within the NIR pulse envelope. To look inside the NIR pulse we use a considerably shorter soft x-ray extreme ultraviolet (XUV) pulse from the Free-Electron Laser in Hamburg (FLASH) as a probe. Our approach relies on the ability of XUV radiation to interrogate core levels of a particular ion and on the sensitivity of the subsequent Auger decay to the valence electron configuration. Due to different interaction mechanisms of pump and probe pulses with the target molecules, and the nonlinearity of the strong field ionization, it was possible to follow ultrafast electron rearrangement with the temporal resolution better than dictated by the convolution of pump and probe pulse profiles.

See also APS Physics Synopsis and DESY News from 13.11.2012

Tracing the transient plasma states with XUV pump-probe spectroscopy

Scheme of experimental geometry
Lupe

We use pump-probe spectroscopy approach to follow the expansion and disintegration dynamics of xenon clusters initiated by the ionization with femtosecond soft x-ray extreme ultraviolet (XUV) pulses from the Free-Electron Laser in Hamburg (FLASH). The ionization by the first XUV pulse leads to the generation of a large number of quasi-free electrons trapped by the space charge of the cluster ions. A temporally delayed, more intense probe pulse of the same wavelength is used to access transient plasma states by tracing the average charge of fragment ions. The results are understood in terms of hydrodynamic expansion model and reveal a timescale for cluster disintegration, which depends essentially on the initial cluster size.

See also J. Phys. B Highlights 2012 Molecular and cluster structure, properties and dynamics

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