Olaf Schwarzkopf

A high-order harmonic generation apparatus for time- and angle-resolved photoelectron spectroscopy

We present a table top setup for time- and angle-resolved photoelectron spectroscopy to investigate band structure dynamics of correlated materials driven far from equilibrium by femtosecond laser pulse excitation. With the electron-phonon equilibration time being in the order of 1-2 ps it is necessary to achieve sub-picosecond time resolution. Few techniques provide both the necessary time and energy resolution to map non-equilibrium states of the band structure. Laser-driven high-order harmonic generation is such a technique. In our experiment, a grating monochromator delivers tunable photon energies up to 40 eV. A photon energy bandwidth of 150 meV and a pulse duration of 100 fs FWHM allow us to cover the k-space necessary to map valence bands at different kz and detect outer core states.

Femtosecond time-resolved photoelectron spectroscopy with a vacuum-ultraviolet photon source based on laser high-order harmonic generation

A laser-based tabletop approach to femtosecond time-resolved photoelectron spectroscopy with photons in the vacuum-ultraviolet (VUV) energy range is described. The femtosecond VUV pulses are produced by high-order harmonic generation (HHG) of an amplified femtosecond Ti:sapphire laser system. Two generations of the same setup and results from photoelectron spectroscopy in the gas phase are discussed. In both generations, a toroidal grating monochromator was used to select one harmonic in the photon energy range of 20-30 eV. The first generation of the setup was used to perform photoelectron spectroscopy in the gas phase to determine the bandwidth of the source. We find that our HHG source has a bandwidth of 140 ± 40 meV. The second and current generation is optimized for femtosecond pump-probe photoelectron spectroscopy with high flux and a small spot size at the sample of the femtosecond probe pulses. The VUV radiation is focused into the interaction region with a toroidal mirror to a spot smaller than 100 × 100 μm(2) and the flux amounts to 10(10) photons/s at the sample at a repetition rate of 1 kHz. The duration of the monochromatized VUV pulses is determined to be 120 fs resulting in an overall pump-probe time resolution of 135 ± 5 fs. We show how this setup can be used to map the transient valence electronic structure in molecular dissociation.

High-resolution constant length Rowland circle monochromator at BESSY

The working principle, the layout and the performance data of a simple Rowland circle soft x-ray monochromator of constant length are described. By translating the grating while scanning the photon energy, defocus, coma and some spherical aberrations vanish, yielding excellent resolution in the optimized energy range between 270 and 590 eV. This result is obtained while using all optical elements at full illumination. The achieved resolution ranks among the best worldwide.

Femtosecond VUV Photon Pulses for Time-resolved Photoelectron Spectroscopy

The potential of time-resolved photoelectron spectroscopy with femtosecond photon pulses in the vacuum ultra violet energy range is explored. Higher harmonics of a femtosecond Ti:Sa laser are generated in rare gases and characterized in terms of intensity, spectral distribution and time structure. This demonstrates the suitability of such a source to study, in real time, changes of the valence bands of molecules during chemical reactions.

Femtosecond XUV photoelectron spectroscopy of ultrafast magnetization dynamics in gadolinium and terbium

The Lanthanide metals gadolium (Gd) and terbium (Tb) are prototypical local-moment ferromagnets. In these systems the magnetic moment derives predominantly from the partial occupancy of the localized 4f core electronic levels. Alignment of the magnetic moments between adjacent atoms in the lattice occurs by spin polarization of the itinerant 5d and 6s valence electrons in an indirect exchange (Ruderman-Kittel-Kasuya-Yosida) interaction.

Real-time Evolution of the Valence Orbitals in a Dissociating Molecule as Revealed by Femtosecond Photoelectron Spectroscopy

We follow in real time the evolution of the valence orbitals of Br2 molecules as the bonds break during dissociation with femtosecond vacuum-ultraviolet (VUV) photoelectron spectroscopy and with simulations of the nuclear and electron dynamics.