PhD Theses

Rotational Coherence Spectroscopy at FLASH: Toward Dynamic Studies in Nanosuperfluids

by Andreas Kickermann

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The field of molecular physics, which is focusing on molecular motion in the transition states of physical, chemical, and biological changes, is a wide-spread research area. It strives to reveal the structural and functional properties of molecules, the chemical bonds between atoms and the time evolution. Many processes occurring in nature upon electronic excitation proceed on the ultrafast femtosecond timescale and can be triggered by modern ultrashort femtosecond-laser sources under laboratory conditions. In the present thesis pump-probe studies were performed to follow molecular motion using ultrashort light pulses in the nanometer wavelength range provided by an XUV free electron laser (FEL). In detail, alignment of molecular species in space under field-free conditions was investigated. In the specific case of rotational wave packets in molecules the rotational dynamics shows characteristic temporal features, which contain a wealth of information on molecular structure and give insight into molecular coupling mechanisms, i.e. rotational constants and transition frequencies.

Within this thesis, Rotational Coherence Spectroscopy (RCS) reveals wave-packet motion observed by subsequent Coulomb explosion of Raman excited carbon monoxide, which results in a time-dependent asymmetry of spatial fragmentation patterns. With the method presented here, the time resolution to elucidate the fast dynamics of strong couplings can be pushed toward a single rotational period even for the fastest rotors. This is due to large pump-probe delays with small subpicosecond step size.

This kind of spectroscopy can also be expanded to molecular species, which are not accessible by other powerful spectroscopic methods, such as Fourier-transform microwave spectroscopy (FTMW). Furthermore, it allows to measure weak molecular couplings on a long timescale (large pump-probe delays), e.g. couplings of molecules in a solution or molecules dissolved in quantum fluids. This is valuable to measure the extraordinary weak couplings in superfluid helium clusters and nanodroplets.



Design and Commissioning of an XUV and Soft X-Ray FEL Pulse Shaper

by Leslie Lamberto Lazzarino

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At the heart of this PhD project is the design of an extreme ultraviolet (XUV) pulse shaper relying on reflective optics. The instrument allows tailoring of the time-frequency spectrum of femtosecond pulses generated by seeded free-electron lasers (FEL) and high-harmonic generation (HHG) sources down to a central wavelength of ~15 nm. The device is based on the geometry of a 4f grating compressor that is a standard concept in ultrafast laser science and technology. It is applied to shorter wavelengths using grazing-incidence optics operated under ultra-high vacuum conditions. The design blaze angle and the line density of the gratings allow the manipulation of all different harmonics typical for seeded FEL and HHG photon sources without the need of realignment of the instrument and even simultaneously in multi-color experiments. A proof-of-principle pulse shaping experiment using 266 nm laser light was performed at the end of the project, demonstrating relative phase-control of femtosecond UV pulses.



Extreme Ultraviolet Fluorescence Spectroscopy of Pure and Core-Shell Rare Gas Clusters at FLASH

by Lasse Schroedter

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The interaction of rare gas clusters with short-wavelength radiation of free-electron lasers (FELs) has been studied extensively over the last decade by means of electron and ion time-of-flight spectroscopy. This thesis describes the design and construction of a fluorescence spectrometer for the extreme ultraviolet (XUV) spectral range and discusses the cluster experiments performed at FLASH, the Free-electron LASer in Hamburg. Fluorescence of xenon and of argon clusters was studied, both in dependence on the FEL pulse intensity and on the cluster size. The FEL wavelength was set to the giant 4d-resonance of xenon at 13.5 nm and the FEL pulse intensity reached peak values of 2.7 · 1015 W/cm2. For xenon clusters, charge states of at least 11+ were identified. For argon, charge states up to 7+ were detected. The cluster-size dependent study revealed a decrease of the fluorescence yield per atom with increasing cluster size. This decrease is explained with the help of a geometric model. It assumes that virtually the entire fluorescence yield stems from shells of ions on the cluster surface, whereas ions in the cluster core predominantly recombine non-radiatively with electrons. However, the detailed analysis of fluorescence spectra from clusters consisting of a core of Xe atoms and a surrounding shell of argon atoms shows that, in fact, a small fraction of the fluorescence signal comes from Xe ions in the cluster core. Interestingly, these ions are as highly charged as the ions in the shells of a pure Xe cluster. This result goes beyond the current understanding of charge and energy transfer processes in these systems and points toward the observation of ultrafast charging dynamics in a time window where mass spectrometry is inherently blind.



Interferometry on small quantum systems at short wavelength

by Sergey Usenko

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The PhD project concentrated on prototypical studies of light-induced correlated manybody dynamics in complex systems. In its course a reflective split-and-delay unit (SDU) for phase-resolved one-color pump-probe experiments with gas phase samples using VUV–XUV laser pulses was built. The collinear propagation of pump and probe pulses is ensured by the special geometry of the SDU and allows to perform phase-resolved (coherent) autocorrelation measurements. The control of the pump-probe delay with attosecond precision is established by a specially developed diagnostic tool based on an in-vacuum white light interferometer that allows to monitor the relative displacement of the SDU reflectors with nanometer resolution. Phase-resolved (interferometric) pump-probe experiments with developed SDU require spatially-resolved imaging of the ionization volume. For this an electron–ion coincidence spectrometer was built. The spectrometer enables coincident detection of photoionization products using velocity map imaging (VMI) technique for electrons and VMI or spatial imaging for ions. In first experiments using the developed SDU and the spectrometer in the ion spatial-imaging mode linear field autocorrelation of free-electron laser pulses at the central wavelength of 38 nm was recorded. A further focus of the work were energy- and time-resolved resonant two-photon ionization experiments using short tunable UV laser pulses on C60 fullerene. The experiments demonstrated that dipole-selective excitation on a timescale faster than the characteristic intramolecular energy dissipation limits the number of accessible excitation pathways and thus results in a narrow resonance. Time-dependent one-color pump-probe study showed that nonadiabatic (vibron) coupling is the dominant energy dissipation mechanism for high-lying electronic excited states in C60.



Generation and Control of Ultrafast 10 µm Laser Pulses for Driving Chemical Dynamics

by Markus Alexander Jakob

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In this thesis, the concept of using a 4 f -line and an acousto-optic modulator mask for temporal pulse shaping was extended to be used at wavelengths around 10 µm. For that purpose, a state-of-the-art optical parametric amplifier (OPA) set-up, driven by a commercial femtosecond laser, has been built to generate broadband pulses in the spectral domain around 10 µm. In the design process various nonlinear crystals were compared theoretically. Home-made diagnostics were used to characterise the generated and temporally shaped laser pulses. The OPA design study revealed the possibility of adapting pump and signal wavelengths used in the generation scheme for broadband amplification, depending on which wavelength the mid-infrared (MIR) idler is tuned to. The OPA set-up is demonstrated to generate carrier-envelope phase stable (CEPstable) pulses with wavelengths between 8–15 µm with bandwidths of 14.6% (5.5 THz) at 8 µm to 16.5% (3.3 THz) at 15 µm. Pulse energies are in the range of several hundreds of nanojoule. The conversion stage is operated in a type I phase matching geometry of a GaSe crystal using intermediate conversion steps. Difference frequency mixing is performed with pump wavelengths of 1.6 µm and signal wavelengths tunable between 1.75–2.1 µm. First measurements indicate higher achievable pulse energies in the MIR using type II phase matching. Pulse shaping was achieved with a spectral resolution of 59GHz at 10.8 µm. The set-up allows manipulation of the MIR laser pulses in a time window of ca. 9 ps at FWHM of a Gaussian intensity pattern. Shaping of the relative CE phase within a pulse train was shown to be applied with a precision of 93 mrad. Temporal pulse shaping via acousto-optic modulation is therefore successfully applied to the MIR spectral range. The shaping capabilities provide excellent opportunities for complex control of quantum systems in their electronic ground state. Choosing resonant photon energies and pulse-shapes allows for highly specific excitations in diverse chemical environments, making the technique interesting for applications ranging from physical chemistry to biology.