Controlling the time-dependent dipole response in liquid-phase targets

This thesis focuses on the dynamical changes in the absorption of liquid-phase targets due to the interaction with strong, ultrashort laser pulses. Therefore, it is shown that the absorption spectrum of aluminum chloride phthalocyanine (AlClPc) in the liquid phase can be dynamically modified through the time- resolved interaction with a second laser pulse, which can be explained by laser- induced coherent coupling dynamics between the ground state and an ensemble of excited states, as reproduced by a few-level toy model. Furthermore, it is shown which time-dependent effects upon the measured absorption spectra are generated due to the presence of the solvent and the non-trivial pulse-form itself, which is confirmed through a second simulation. The presented results contribute to a better understanding in how intense laser fields interact with complex molecules in solution and can, in the future, be used to improve dynamic laser-control of complex systems

XUV pump–XUV probe transient absorption spectroscopy at FELs

The emergence of ultra-intense extreme-ultraviolet (XUV) and X-ray free-electron lasers (FELs) has opened the door for the experimental realization of non-linear XUV and X-ray spectroscopy techniques. Here we demonstrate an experimental setup for an all-XUV transient absorption spectroscopy method for gas-phase targets at the FEL. The setup combines a high spectral resolving power of $E/ΔE$ ≈ 1500 with sub-femtosecond interferometric resolution, and covers a broad XUV photon-energy range between approximately 20 and 110 eV. We demonstrate the feasibility of this setup firstly on a neon target. Here, we intensity- and time-resolve key aspects of non-linear XUV-FEL light–matter interactions, namely the non-resonant ionization dynamics and resonant coupling dynamics of bound states, including XUV-induced Stark shifts of energy levels. Secondly, we show that this setup is capable of tracking the XUV-initiated dissociation dynamics of small molecular targets (oxygen and diiodomethane) with site-specific resolution, by measuring the XUV transient absorption spectrum. In general, benefitting from a single-shot detection capability, we show that the setup and method provides single-shot phase-locked XUV pulse pairs. This lays the foundation to perform, in the future, experiments as a function of the XUV interferometric time delay and the relative phase, which enables advanced coherent non-linear spectroscopy schemes in the XUV and X-ray spectral range

XUV pump-XUV probe transient absorption spectroscopy at FELs

The emergence of ultra-intense extreme-ultraviolet (XUV) and X-ray free-electron lasers (FELs) has opened the door for the experimental realization of non-linear XUV and X-ray spectroscopy techniques. Here we demonstrate an experimental setup for an all-XUV transient absorption spectroscopy method for gas-phase targets at the FEL. The setup combines a high spectral resolving power of E/ΔE ≈ 1500 with sub-femtosecond interferometric resolution, and covers a broad XUV photon-energy range between approximately 20 and 110 eV. We demonstrate the feasibility of this setup firstly on a neon target. Here, we intensity- and time-resolve key aspects of non-linear XUV-FEL light-matter interactions, namely the non-resonant ionization dynamics and resonant coupling dynamics of bound states, including XUV-induced Stark shifts of energy levels. Secondly, we show that this setup is capable of tracking the XUV-initiated dissociation dynamics of small molecular targets (oxygen and diiodomethane) with site-specific resolution, by measuring the XUV transient absorption spectrum. In general, benefitting from a single-shot detection capability, we show that the setup and method provides single-shot phase-locked XUV pulse pairs. This lays the foundation to perform, in the future, experiments as a function of the XUV interferometric time delay and the relative phase, which enables advanced coherent non-linear spectroscopy schemes in the XUV and X-ray spectral range.ISSN:1359-6640ISSN:1364-549

XUV-Initiated Dissociation Dynamics of Molecular Oxygen (O2_2)

We performed a time-resolved spectroscopy experixment on the dissociation of oxygen molecules after the interactionwith intense extreme-ultraviolet (XUV) light from the free-electronlaser in Hamburg at Deutsches Elektronen-Synchrotron. Using anXUV-pump/XUV-probe transient-absorption geometry with asplit-and-delay unit, we observe the onset of electronic transitionsin the O$^{2+}$ cation near 50 eV photon energy, marking the end ofthe progression from a molecule to two isolated atoms. We observetwo different time scales of 290 ± 53 and 180 ± 76 fs for theemergence of different ionic transitions, indicating differentdissociation pathways taken by the departing oxygen atoms.With regard to the emerging opportunities of tuning the centralfrequencies of pump and probe pulses and of increasing the probe−pulse bandwidth, future pump−probe transient-absorptionexperiments are expected to provide a detailed view of the coupled nuclear and electronic dynamics during molecular dissociatio

Strong-Field Extreme-Ultraviolet Dressing of Atomic Double Excitation

We report on the experimental observation of a strong-field dressing of an autoionizing two-electronstate in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetricFano line shape of this transition is spectrally resolved, and we observe modifications of the resonanceasymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules.A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, drivenby classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phaseshifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. Theexperimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transientenergy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way ofperforming nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronictransitions in atoms at short wavelengths

Nonlinear Coherence Effects in Transient-Absorption Ion Spectroscopy with Stochastic Extreme-Ultraviolet Free-Electron Laser Pulses

We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to $2p-3d$ bound–bound transitions between the spin-orbit multiplets $^3P_{0,1,2}$ and $^3D_{1,2,3}$ of the transiently produced doubly charged Ne$^{2+}$ ion are revealed, with time-dependent spectral changes over a time-delay range of (2.4±0.3) fs. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the ac Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron laser radiation when the phase-locked pump and probe pulses precisely overlap in time

Measuring the frequency chirp of extreme-ultraviolet free-electron laser pulses by transient absorption spectroscopy

High-intensity ultrashort pulses at extreme ultraviolet (XUV) and x-ray photon energies, delivered by state-of-the-art free-electron lasers (FELs), are revolutionizing the field of ultrafast spectroscopy. For crossing the next frontiers of research, precise, reliable and practical photonic tools for the spectro-temporal characterization of the pulses are becoming steadily more important. Here, we experimentally demonstrate a technique for the direct measurement of the frequency chirp of extreme-ultraviolet free-electron laser pulses based on fundamental nonlinear optics. It is implemented in XUV-only pump-probe transient-absorption geometry and provides in-situ information on the time-energy structure of FEL pulses. Using a rate-equation model for the time-dependent absorbance changes of anionized neon target, we show how the frequency chirp can be directly extracted and quantified from measured data. Since the method does not rely on an additional external field, we expect a widespread implementation at FELs benefiting multiple science fields by in-situ on-target measurement and optimization of FEL-pulse properties

All-XUV Pump-Probe Transient Absorption Spectroscopy of the Structural Molecular Dynamics of Di-iodomethane

In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite the I: 4$d_{5/2}–σ^∗$ transition of a gas-phase di-iodomethane (CH$_2$I$_2$) target. This site-specific excitation generates a 4$d$ core hole located at an iodine site, which leaves the molecule in a well-defined excited state. We subsequently measure the time-dependent absorption change of the molecule with the FEL probe spectrum centered on the same I: 4$_d$ resonance. Using ab initio calculations of absorption spectra of a transient isomerization pathway observed in earlier studies, our time-resolved measurements allow us to assign the timescales of the previously reported direct and indirect dissociation pathways. The presented method is thus sensitive to excited-state molecular geometries in a time-resolved manner, following a core-resonant site-specific trigger