Martin Wilke

Monochromatization of femtosecond XUV light pulses with the use of reflection zone plates.

We report on a newly built laser-based tabletop setup which enables generation of femtosecond light pulses in the XUV range employing the process of high-order harmonic generation (HHG) in a gas medium. The spatial, spectral, and temporal characteristics of the XUV beam are presented. Monochromatization of XUV light with minimum temporal pulse distortion is the central issue of this work. Off-center reflection zone plates are shown to be advantageous when selection of a desired harmonic is carried out with the use of a single optical element. A cross correlation technique was applied to characterize the performance of the zone plates in the time domain. By using laser pulses of 25 fs length to pump the HHG process, a pulse duration of 45 fs for monochromatized harmonics was achieved in the present setup.

Time-of-flight electron spectrometer for a broad range of kinetic energies.

A newly constructed time-of-flight electron spectrometer of the magnetic bottle type is characterized for electron detection in a broad range of kinetic energies. The instrument is designed to measure the energy spectra of electrons generated from liquids excited by strong laser fields and photons in the range of extreme ultra violet and soft X-rays. Argon inner shell electrons were recorded to calibrate the spectrometer and investigate its characteristics, such as energy resolution and collection efficiency. Its energy resolution ΔE/E of 1.6% allows resolving the Ar 2p spin orbit structure at kinetic energies higher than 100 eV. The collection efficiency is determined and compared to that of the spectrometer in its field-free configuration.

Ultrafast photoelectron spectroscopy of solutions: space-charge effect

The method of time-resolved XUV photoelectron spectroscopy is applied in a pump–probe experiment on a liquid micro-jet. We investigate how the XUV energy spectra of photoelectrons are influenced by the space charge created due to ionization of the liquid medium by the pump laser pulse. XUV light from high-order harmonic generation is used to probe the electron population of the valence shell of iron hexacyanide in water. By exposing the sample to a short UV pump pulse of 266 nm wavelength and ~55 fs duration, we observe an energy shift of the spectral component associated with XUV ionization from the Fe 3d(t2g) orbital as well as a shift of the water spectrum. Depending on the sequence of the pump and probe pulses, the arising energy shift of photoelectrons acquires a positive or negative value. It exhibits a sharp positive peak at small time delays, which facilitates to determine the temporal overlap between pump and probe pulses. The negative spectral shift is due to positive charge accumulated in the liquid medium during ionization. Its dissipation is found to occur on a (sub)nanosecond time scale and has a biexponential character. A simple mean-field model is provided to interpret the observations. A comparison between the intensity dependencies of the spectral shift and the UV ionization yield shows that the space-charge effect can be significantly reduced when the pump intensity is attenuated below the saturation level of water ionization. For the given experimental conditions, the saturation intensity lies at W cm−2.

Charge Transfer Dynamics at Dye-Sensitized ZnO and TiO2 Interfaces Studied by Ultrafast XUV Photoelectron Spectroscopy.

Interfacial charge transfer from photoexcited ruthenium-based N3 dye molecules into ZnO thin films received controversial interpretations. To identify the physical origin for the delayed electron transfer in ZnO compared to TiO2, we probe directly the electronic structure at both dye-semiconductor interfaces by applying ultrafast XUV photoemission spectroscopy. In the range of pump-probe time delays between 0.5 to 1.0u2009ps, the transient signal of the intermediate states was compared, revealing a distinct difference in their electron binding energies of 0.4u2009eV. This finding strongly indicates the nature of the charge injection at the ZnO interface associated with the formation of an interfacial electron-cation complex. It further highlights that the energetic alignment between the dye donor and semiconductor acceptor states appears to be of minor importance for the injection kinetics and that the injection efficiency is dominated by the electronic coupling.

Ultrafast Spin Crossover in [FeII(bpy)3]2+: Revealing Two Competing Mechanisms by Extreme Ultraviolet Photoemission Spectroscopy

Photoinduced spin-flip in FeII complexes is an ultrafast phenomenon that has the potential to become an alternative to conventional processing and magnetic storage of information. Following the initial excitation by visible light into the singlet metal-to-ligand charge-transfer state, the electronic transition to the high-spin quintet state may undergo different pathways. Here we apply ultrafast XUV (extreme ultraviolet) photoemission spectroscopy to track the low-to-high spin dynamics in the aqueous iron tris-bipyridine complex, [Fe(bpy)3 ]2+ , by monitoring the transient electron density distribution among excited states with femtosecond time resolution. Aided by first-principles calculations, this approach enables us to reveal unambiguously both the sequential and direct de-excitation pathways from singlet to quintet state, with a branching ratio of 4.5:1.

Ultrafast excited states dynamics of [Ru(bpy)3]2+ dissolved in ionic liquids

Room-temperature ionic liquids (ILs) represent a well-known class of materials exhibiting extremely low vapor pressures and high electrochemical stability. These properties make ILs attractive for various applications requiring UHV conditions. Here, we apply 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM][TfO] as a solvent to investigate the excited state dynamics of the transition metal complex [Ru(bpy)3]2+ with the use of ultrafast XUV photoelectron spectroscopy. This study is aimed to reveal the effect of the IL environment when the frontier molecular orbitals and the states dynamics of the solute need to be addressed. By initiating the electron dynamics with a pump laser pulse of 480 nm wavelength, we can unambiguously characterize the kinetics of the excited states of [Ru(bpy)3]2+ and determine their absolute binding energies. From a global fit analysis of the transient signal, the binding energies of the initially populated metal-to-ligand charge-transfer state 1MLCT and the thermally relaxed 3MLCT are inferred to be -0.2 eV and 0.3 eV, respectively. A three-state model, including the intersystem crossing (ISC) from the 1MLCT to the 3MLCT state and the intramolecular vibrational relaxation (IVR) within the triplet configuration, is used to describe the involved decay processes. The kinetic constants of (37 ± 10) fs for the ISC and (120 ± 20) fs for the IVR are found to be in agreement with the values previously reported for aqueous solution. The obtained results open up exciting new possibilities in the field of liquid phase spectroscopy.

Reply to the ‘Comment on “Charge Transfer to Solvent Dynamics in Iodide Aqueous Solution Studied at Ionization Threshold”’ by A. Lübcke and H.-H. Ritze, Phys. Chem. Chem. Phys., 2015, 17, DOI: 10.1039/C5CP00346F

The role of experimental conditions in the study of the early-time charge transfer to solvent dynamics in iodide aqueous solution is revised. Under the short (∼50 fs) laser pulse regime of the current experiment, the presence of the pump–probe cross-correlation signal in the transient photoelectron spectra can be ruled out due to the much larger time scale of the electron-transfer dynamics. The ratio of the ionization yields from different initial states of iodide and water is argued to be dependent on the electron kinetic energy, and to be influenced by the presence of a bound resonance state above the vacuum threshold. Re-evaluation of our experimental data reassures the presence of an intermediate state in the charge-transfer process, initiated by electronic excitation into the continuum spectrum.

Laser-assisted electron scattering in strong-field ionization of dense water vapor by ultrashort laser pulses

We report on strong-field ionization of dense water gas in a short infrared laser pulse. By employing a unique combination of photoelectron spectroscopy with a liquid micro-jet technique, we observe how the character of electron emission at high kinetic energies changes with the increase of the medium density. This change is associated with the process of laser-assisted electron scattering (LAES) on neighboring particles, which becomes a dominant mechanism of hot electron emission at higher medium densities. The manifestation of this mechanism is found to require densities that are orders of magnitude lower than those considered for heating the laser-generated plasmas via the LAES process. The experimental results are supported by simulations of the LAES yield with the use of the Kroll–Watson theory.