Solvated electrons in liquid water are one of the seemingly simplest, but most important, transients in chemistry and biology, but they have resisted disclosing important information about their energetics, binding motifs and dynamics. Here we report the first ultrafast liquid-jet photoelectron spectroscopy measurements of solvated electrons in liquid water. The results prove unequivocally the existence of solvated electrons bound at the water surface and of solvated electrons in the bulk solution, with vertical binding energies of 1.6 eV and 3.3 eV, respectively, and with lifetimes longer than 100 ps. The unexpectedly long lifetime of solvated electrons bound at the water surface is attributed to a free-energy barrier that separates surface and interior states. Beyond constituting important energetic and kinetic benchmark and reference data, the results also help to understand the mechanisms of a number of very efficient electron-transfer processes in nature.

Electron spectroscopy for chemical analysis (ESCA) is a powerful tool for the quantitative analysis of the composition and the chemical environment of molecular systems. Due to the lack of compatibility of liquids and vacuum, liquid-phase ESCA is much less well established. The chemical shift in the static ESCA approach is a particularly powerful observable quantity for probing electron orbital energies in molecules in different molecular environments. Employing high harmonics of 800 nm (40 eV), near-infrared femtosecond pulses, and liquid-water microbeams in vacuum we were able to add the dimension of time to the liquid interface ESCA technique. Tracing time-dependent chemical shifts and energies of valence electrons in liquid interfacial water in time, we have investigated the timescale and molecular signatures of laser-induced liquid-gas phase transitions on a picosecond timescale.

Electron spectroscopy for chemical analysis (ESCA) being conceptually a photoelectron spectroscopy is established as a chemically specific probe mostly for surface analysis. Liquid phase ESCA for volatile liquids has become possible through the development of the liquid microjet technique in vacuum enabling the measurement of liquid interface photoelectron emission at the high vapor pressure of volatile liquids. Recently we have been able to add the dimension of time to the liquid interface ESCA technique employing high-harmonics soft X-ray and UV/near IR femtosecond pulses in combination with liquid water micro beams in vacuum. The concepts as well as technical details are outlined and several characteristic applications are high-

Selectively excited benzene and toluene in the gas and solution phase have been investigated with ultrafast transient absorption spectroscopy to study the impact of a solvent on the time scales of intramolecular vibrational energy redistribution (IVR). It has been found that multiple time scales exist for isolated benzene (toluene) in agreement with theory. A comparison of gas-phase and solution experiments revealed the effect and magnitude of solvent assisted IVR. Although the ultrafast IVR component is hardly influenced by the solvent, the picosecond time scale of IVR appears to be contracted in solution with respect to the gas phase due to interactions (collisions) with the solvent and an overall acceleration of slower IVR components. In addition, we find that an internal rotor (i.e., a methyl group on an aromatic ring) accelerates IVR in the gas phase significantly whereas the effect appears to be largely concealed in solution.

Femtosecond pump probe spectroscopy was employed to measure intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of benzene in the gas phase and in supercritical (sc) CO 2. We observe two IVR time scales the faster of which proceeds within s ð1Þ IVR < 0:5 ps. The slower IVR component has a time constant of s ð2Þ IVR ¼ ð48 AE 5Þ ps in the gas phase and in scCO 2 is accelerated by interactions with the solvent. At the highest CO 2 density it is reduced to s ð2Þ IVR ¼ ð6 AE 1Þ ps. The corresponding IVR rate constants show a similar density dependence as the VET rate constants. Model calculations suggest that both quantities correlate with the local CO 2 density in the immediate surrounding of the benzene molecule.

Supercritical water and methanol have recently drawn much attention in the field of green chemistry. It is crucial to an understanding of supercritical solvents to know their dynamics and to what extent hydrogen (H) bonds persist in these fluids. Here, we show that with femtosecond infrared (IR) laser pulses water and methanol can be heated to temperatures near and above their critical temperature T c and their molecular dynamics can be studied via ultrafast photoelectron spectroscopy at liquid jet interfaces with high harmonics radiation. As opposed to previous studies, the main focus here is the comparison between the hydrogen bonded systems of methanol and water and their interpretation by theory. Superheated water initially forms a dense hot phase with spectral features resembling those of monomers in gas phase water. On longer timescales, this phase was found to build hot aggregates, whose size increases as a function of time. In contrast, methanol heated to temperatures near T c initially forms a broad distribution of aggregate sizes and some gas. These experimental features are also found and analyzed in extended molecular dynamics simulations. Additionally, the simulations enabled us to relate the origin of the different behavior of these two hydrogen-bonded liquids to the nature of the intermolecular potentials. The combined experimental and theoretical approach delivers new insights into both superheated phases and may contribute to understand their different chemical reactivities.

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Fast processes at liquid interfaces play a crucial role in various fields of reaction dynamics. 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.