Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser

In pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the time-dependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment

Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera

Citation: Amini, K., Boll, R., Lauer, A., Burt, M., Lee, J. W. L., Christensen, L., . . . Rolles, D. (2017). Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera. Journal of Chemical Physics, 147(1). doi:10.1063/1.4982220Laser-induced adiabatic alignment and mixed-field orientation of 2,6-difluoroiodobenzene (C6H3F2I) molecules are probed by Coulomb explosion imaging following either near-infrared strong-field ionization or extreme-ultraviolet multi-photon inner-shell ionization using free-electron laser pulses. The resulting photoelectrons and fragment ions are captured by a double-sided velocity map imaging spectrometer and projected onto two position-sensitive detectors. The ion side of the spectrometer is equipped with a pixel imaging mass spectrometry camera, a time-stamping pixelated detector that can record the hit positions and arrival times of up to four ions per pixel per acquisition cycle. Thus, the time-of-flight trace and ion momentum distributions for all fragments can be recorded simultaneously. We show that we can obtain a high degree of one-and three-dimensional alignment and mixed-field orientation and compare the Coulomb explosion process induced at both wavelengths. © 2017 Author(s)

Ultraschnelle Beugung und Abbildung mit ionisierte Elektronen

This thesis investigates two different techniques that can potentially be used for the direct imaging of ultrafast structural dynamics of molecules at the femtosecond time scale and with Angstrom spatial resolution. Conventional ultrafast diffraction and imaging techniques use elastically scattered energetic particles, i.e. X-rays or electrons, to probe the molecular structure. The two techniques studied in this thesis make use of inelastic processes to produce ionized (secondary) electrons, that are subsequently used to image the molecular structure. The first technique studied in this thesis is Laser Induced Electron Diffraction (LIED). The ionization of a molecule in a strong, low-frequency laser field leads to the creation of a photoelectron wavepacket that is driven by the laser and can re-collide with its parent molecule. These re-scattered photoelectrons display diffraction features that can be used for the reconstruction of the molecular structure. A series of experiments was performed, investigating the effect of the molecular frame on the Photoelectron Angular Distribution (PAD) of impulsively aligned and strong-field ionized CF3I molecules using a Velocity Map Imaging Spectrometer (VMIS). It is shown that using the impulsive laser alignment technique enables taking differential measurements that bring out directly and clearly LIED effects in the PAD, even for a relatively complex molecule such as CF3I. The comparison of the experimental results at different laser intensities and at two different probe wavelengths, i.e. 800 and 1300 nm, shows that the LIED effect is robust and reproducible for a wide range of experimental conditions, and at comparatively low re-collision energies. Moreover, the first results from Time-Dependent Density Functional Theory (TDDFT) calculations indicate that the ionization of multiple molecular orbitals, which have a distinct shape and orientation with respect to the molecular frame, leads to significant effects and can be identified in the experimental results. The second technique investigated in this thesis proposes the use of secondary electrons produced by electron impact ionization for the imaging of the molecular structure during a dynamical process. Specifically, Impact Ionized Coherent Electron Emission (IICEE), which leads to the interference between electrons that are ejected from two identical atomic centres within a molecule, was investigated experimentally. A commercially available table-top Ultrafast Electron Diffraction (UED) source was used to produce a beam of (primary) electrons that subsequently ionizes the target atom or molecule and generates energetic secondary electrons. The table-top UED source was combined with a high-energy Velocity Map Imaging Spectrometer (VMIS) and applied to the study of secondary electron emission. By comparing the spectra of Helium and H2 to theoretical calculations, it is shown that hints of IICEE effects due to the molecular structure of H2 may be visible in the experimental data. However, possible systematic errors in the experiment and the shortcomings of the theoretical model in reproducing the low-energy part of the spectrum make an unambiguous assignment to IICEE effects difficult. Simulations with perfectly aligned and partially aligned H2 were used to illustrate the effect of alignment on the secondary electron spectrum. It is shown that using molecular alignment enables clear and unambiguous extraction of molecular effects from secondary electron spectra of impact ionized molecules, in a similar fashion as demonstrated by the LIED experiments.Diese Arbeit untersucht zwei verschiedene Techniken, welche potenziell zur direkten Abbildung von ultraschneller Strukturdynamik von Molekülen auf der Zeitskala von Femtosekunden und mit Angström räumlicher Auflösung verwendet werden können. Konventionelle ultraschnelle Beugungs- und Abbildungstechniken verwenden elastisch gestreute energetische Teilchen, d.h. Röntgenstrahlen oder Elektronen, um die molekulare Struktur zu untersuchen. Die in dieser Arbeit untersuchten Techniken nutzen inelastische Prozesse um ionisierte (sekundäre) Elektronen zu erzeugen, mittels derer anschließend die molekulare Struktur abgebildet wird. Die erste Technik, die in dieser Arbeit untersucht wird, ist Laser Induced Electron Diffraction (LIED). Die Ionisation eines Moleküls in einem starken, niederfrequenten Laserfeld führt zur Erzeugung eines Photoelektronenwellenpakets, welches durch den Laser beschleuningt wird und mit dem ursprünglichen Molekül wieder kollidieren kann. Diese gestreuten Photoelektronen zeigen Beugungsmerkmale, d.h. LIED-Muster, welche für die Rekonstruktion der molekularen Struktur verwendet werden können. Es wurde eine Reihe von Experimenten durchgeführt, in denen der Einfluss der molekularen Struktur auf die Photoelektronen-Winkelverteilung von impulsiv ausgerichteten CF3I Molekülen untersucht wurde. Die Technik der impulsiven Laserausrichtung ermöglicht Differenzmessungen, mittels derer direkt und eindeutig LIED-Effekte in der Photoelektronen-Winkelverteilung hervorgehoben werden, auch f\"{u}r ein relativ komplexes Molekül wie CF3I. Der Vergleich der Versuchsergebnisse bei verschiedenen Laserintensitäten sowie bei zwei verschiedenen Abfrage- Wellenlängen von 800 nm und 1300 nm zeigt, dass der LIED-Effekt für einen weiten Bereich von experimentellen Bedingungen und bei vergleichsweise niedrigen Wiederkollisionsenergien robust und reproduzierbar ist. Darüber hinaus zeigen erste Ergebnisse von Berechnungen zeitabhängiger Dichtefunktionaltheorie, dass die Ionisation von mehreren Molekülorbitalen, die eine unterschiedliche Form und Ausrichtung in Bezug auf das Molekül aufweisen, ebenfalls signifikant ist und in den Versuchsergebnissen identifiziert werden kann. Die zweite in dieser Arbeit untersuchte Technik basiert auf dem Vorschlag Sekundärelektronen, welche durch Elektronenstoß- Ionisation erzeugt werden, zum Abbilden der Molekülstruktur während eines dynamischen Prozesses zu verwenden. Insbesondere wurden Interferenzeffekte zwischen Elektronen, welche mittels kohärenter Stoßionisation aus zwei unterschiedlichen Atomzentren innerhalb eines Moleküls ausgestoßen werden, experimentell untersucht. Eine kommerziell erhältliche ultraschnelle Elektronenquelle wurde verwendet um einen Primärstrahl von Elektronen zu erzeugen, welcher das Atom oder Molekül ionisiert und energetische Sekundärelektronen erzeugt. Durch den Vergleich der Spektren von Helium und H2 mit theoretischen Berechnungen wird gezeigt, dass Hinweise auf Interferenzeffekte in den experimentellen Daten sichtbar sind, die von der molekularen Struktur von H2 herrühren. Allerdings erschweren mögliche systematische Fehler im Experiment sowie Mängel des theoretischen Modells im niederenergetischen Teil des Spektrums eine eindeutige Zuordnung dieser Effekte zur molekularen Struktur. Simulationen mit perfekt und teilweise ausgerichteten H2-Molekülen wurden verwendet, um zu zeigen dass molekulare Effekte in den Sekundärelektronenspektren von ionisierten Molekülen klar und eindeutig mittels molekularer Ausrichtung extrahiert werden können

Photodissociation of Aligned CH3I\mathrm{CH_3I} and C6H3F2I\mathrm{C_{6}H_{3}F_{2}I} Molecules Probed with Time-Resolved Coulomb Explosion Imaging by Site-Selective XUV Ionization

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon–iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules

Photodissociation of aligned CH3I and C6H3F2I molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization.

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon-iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.peerReviewe