Femtosekunden-Photoelektronenspektroskopie mit extrem ultravioletter Strahlung an Flüssigkeitsgrenzflächen

Abstract

In this work time-resolved photoelectron spectroscopy with femtosecond laser pulses in the extrem ultraviolett has been applied to liquid interfaces for the first time. Thereby laser induced phase transitions in water, methanol, and ethanol could be investigated. The solvents were heated by a short laser pulse in the infrared spectral region (2.6 - 3.0 µm) by excitation of the OH-stretch vibrations. Subsequently the 25th harmonic of a 800 nm Ti:Sa-laser pulse was used to monitor time-resolved photoelectron spectra of the evolving system.My means of this new method the spectral signature and the time scales of such a laser driven phase transition could be revealed. In water energy dependent profiles were obtained which provide insight into the molecular processes involved in an ultrafast phase transition. It was shown that only moderate heating led to a homogenous transformation from the liquid to the gaseous phase. At high energies inhomogeneities arose which supported the development of a mixed gas-cluster-phase. Due to the comparably low energy deposition in alcohols only homogenous phase transitions were shown under the experimental conditions.The bi-exponential decrease of intensity in the photoelectron spectra of the liquid correlate with molecular dynamical calculations which show a bi-exponential decrease of the density at the heated surface. The fast time constant of the phase transition tends to show no energy dependence at values of several picoseconds. In contrast the slower second time constant shows a distinct energy dependence and lies in the range of 10 to 100 ps.The spectral changes in the photoelectron spectra suggest a superposition of two major contributions: the polarization of the surrounding molecules and the orbital interference promoted by the hydrogen-bonded network. A fast shift occurring at all excitation energies could reveal the contribution of the hydrogen bonds separately.Besides the time-resolved experiments also polarization-dependent measurements were carried out. For the first time the anisotropy parameter β of the 1b1-Orbital of liquid water could be measured directly. A value of 0.95 ± 0.1 at a photon energy of 38.6 eV was received (gas phase: 1.38).This experiment is a fundamental step towards time-resolved ESCA (Electron spectroscopy for chemical analysis) at the liquid phase. The determination of chemical shifts in time can deliver detailed information on transient electron densities at chemical reactions.High-harmonic generation in combination with the micro liquid beam offers excellent opportunities for investigating chemical reactions at the liquid-vacuum-interface. Such experiments exhibit a high potential for elucidating processes relevant in atmospheric chemistry.