Bastian Manschwetus

Two-photon double ionization of neon using an intense attosecond pulse train

We present a demonstration of two-photon double ionization of neon using an intense extreme ultraviolet (XUV) attosecond pulse train (APT) in a photon energy regime where both direct and sequential mechanisms are allowed. For an APT generated through high-order harmonic generation (HHG) in argon we achieve a total pulse energy close to 1μJ, a central energy of 35 eV, and a total bandwidth of ∼30 eV. The APT is focused by broadband optics in a neon gas target to an intensity of 3×1012Wcm−2. By tuning the photon energy across the threshold for the sequential process the double ionization signal can be turned on and off, indicating that the two-photon double ionization predominantly occurs through a sequential process. The demonstrated performance opens up possibilities for future XUV-XUV pump-probe experiments with attosecond temporal resolution in a photon energy range where it is possible to unravel the dynamics behind direct versus sequential double ionization and the associated electron correlation effects.

Self-Probing Spectroscopy of the SF6 Molecule: A Study of the Spectral Amplitude and Phase of the High Harmonic Emission

We present characterizations of the attosecond pulse train produced in the high harmonic generation (HHG) from SF6 molecules irradiated by a strong pulsed laser field at 800 nm. At harmonic order 17, we observe a minimum in the amplitude of the emitted spectrum and a corresponding distortion in the phase. Our experimental results are compared to two models: a multicenter interference model focused on the effect of the structure of the SF6 molecule in HHG and a model focused on the interferences between multiple ionization channels in HHG. We find that the experimental results agree very well with the multiple ionization channels model, illustrating that HHG in molecules can be very complex and that it provides insights of the intramolecular electron dynamics during the interaction process.

Coulomb-explosion imaging of concurrent CH2BrI photodissociation dynamics.

The dynamics following laser-induced molecular photodissociation of gas-phase CH2BrI at 271.6 nm were investigated by time-resolved Coulomb-explosion imaging using intense near-IR femtosecond laser pulses. The observed delay-dependent photofragment momenta reveal that CH2BrI undergoes C-I cleavage, depositing 65.6% of the available energy into internal product states, and that absorption of a second UV photon breaks the C-Br bond of CH2Br. Simulations confirm that this mechanism is consistent with previous data recorded at 248 nm, demonstrating the sensitivity of Coulomb-explosion imaging as a real-time probe of chemical dynamics.

Design and test of a broadband split-and-delay unit for attosecond XUV-XUV pump-probe experiments

We present the design of a split-and-delay unit for the production of two delayed replicas of an incident extreme ultraviolet (XUV) pulse. The device features a single grazing incidence reflection in combination with attenuation of remaining infrared light co-propagating with the XUV beam, offering a high throughput without the need of introducing additional optics that would further decrease the XUV flux. To achieve the required spatial and temporal stabilities, the device is controlled by two PID-controllers monitoring the delay and the beam pointing using an optical reference laser beam, making collimation of the beam by additional optics unnecessary. Finally, we demonstrate the stability of the split-and-delay unit by performing all-reflective autocorrelation measurements on broadband few-cycle laser pulses.

Time-resolved inner-shell photoelectron spectroscopy : From a bound molecule to an isolated atom

Due to its element and site specificity, inner-shell photoelectron spectroscopy is a widely used technique to probe the chemical structure of matter. Here, we show that time-resolved inner-shell photoelectron spectroscopy can be employed to observe ultrafast chemical reactions and the electronic response to the nuclear motion with high sensitivity. The ultraviolet dissociation of iodomethane (CH3I) is investigated by ionization above the iodine 4d edge, using time-resolved inner-shell photoelectron and photoion spectroscopy. The dynamics observed in the photoelectron spectra appear earlier and are faster than those seen in the iodine fragments. The experimental results are interpreted using crystal-field and spin-orbit configuration interaction calculations, and demonstrate that time-resolved inner-shell photoelectron spectroscopy is a powerful tool to directly track ultrafast structural and electronic transformations in gas-phase molecules.

CAMP@FLASH: an end-station for imaging, electron- and ion-spectroscopy, and pump–probe experiments at the FLASH free-electron laser

Beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics for the installation of the new permanent CAMP end-station, a multi-purpose instrument optimized for electron- and ion-spectroscopy, imaging and pump–probe experiments. An overview of the layout, beam transport, focusing capabilities, and experimental possibilities of this new end-station, as well as results from its commissioning and first experiments, are presented.

Micro-Focusing of Broadband High-Order Harmonic Radiation by a Double Toroidal Mirror

We present an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband XUV radiation. Optimization of the focusing optics alignment is carried out with the aid of an XUV wavefront sensor. Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6

Amplitude and Phase of Resonant High Harmonic Emission in SF6

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An Experimental Protocol for Femtosecond NIR/UV - XUV Pump-Probe Experiments with Free-Electron Lasers

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Macroscopic Optimization of High Harmonic Generation for High Power Laser Pulses

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