Two-Center Interference in the Photoionization Delays of Kr_{2}.

We present the experimental observation of two-center interference in the ionization time delays of Kr_{2}. Using attosecond electron-ion-coincidence spectroscopy, we simultaneously measure the photoionization delays of krypton monomer and dimer. The relative time delay is found to oscillate as a function of the electron kinetic energy, an effect that is traced back to constructive and destructive interference of the photoelectron wave packets that are emitted or scattered from the two atomic centers. Our interpretation of the experimental results is supported by solving the time-independent Schrödinger equation of a 1D double-well potential, as well as coupled-channel multiconfigurational quantum-scattering calculations of Kr_{2}. This work opens the door to the study of a broad class of quantum-interference effects in photoionization delays and demonstrates the potential of attosecond coincidence spectroscopy for studying weakly bound systems.

Different timescales during ultrafast stilbene isomerization in the gas and liquid phases revealed using time-resolved photoelectron spectroscopy.

Directly contrasting ultrafast excited-state dynamics in the gas and liquid phases is crucial to understanding the influence of complex environments. Previous studies have often relied on different spectroscopic observables, rendering direct comparisons challenging. Here, we apply extreme-ultraviolet time-resolved photoelectron spectroscopy to both gaseous and liquid cis-stilbene, revealing the coupled electronic and nuclear dynamics that underlie its isomerization. Our measurements track the excited-state wave packets from excitation along the complete reaction path to the final products. We observe coherent excited-state vibrational dynamics in both phases of matter that persist to the final products, enabling the characterization of the branching space of the S1-S0 conical intersection. We observe a systematic lengthening of the relaxation timescales in the liquid phase and a red shift of the measured excited-state frequencies that is most pronounced for the complex reaction coordinate. These results characterize in detail the influence of the liquid environment on both electronic and structural dynamics during a complete photochemical transformation.

Attosecond spectroscopy of size-resolved water clusters.

Electron dynamics in water are of fundamental importance for a broad range of phenomena1-3, but their real-time study faces numerous conceptual and methodological challenges4-6. Here we introduce attosecond size-resolved cluster spectroscopy and build up a molecular-level understanding of the attosecond electron dynamics in water. We measure the effect that the addition of single water molecules has on the photoionization time delays7-9 of water clusters. We find a continuous increase of the delay for clusters containing up to four to five molecules and little change towards larger clusters. We show that these delays are proportional to the spatial extension of the created electron hole, which first increases with cluster size and then partially localizes through the onset of structural disorder that is characteristic of large clusters and bulk liquid water. These results indicate a previously unknown sensitivity of photoionization delays to electron-hole delocalization and indicate a direct link between electronic structure and attosecond photoionization dynamics. Our results offer new perspectives for studying electron-hole delocalization and its attosecond dynamics.

Intermolecular Coulombic Decay in Liquid Water.

We report the first observation of intermolecular Coulombic decay (ICD) in liquid water following inner-valence ionization. By combining a monochromatized tabletop high-harmonic source with a liquid microjet, we record electron-electron coincidence spectra at two photon energies that identify the ICD electrons, together with the photoelectrons originating from the 2a_{1} inner-valence band of liquid water. Our results confirm the importance of ICD as a source of low-energy electrons in bulk liquid water and provide quantitative results for modeling the velocity distribution of the slow electrons that are thought to dominate radiation damage in aqueous environments.

Attosecond interferometry of shape resonances in the recoil frame of CF4.

Shape resonances play a central role in many areas of science, but the real-time measurement of the associated many-body dynamics remains challenging. Here, we present measurements of recoil frame angle-resolved photoionization delays in the vicinity of shape resonances of CF4. This technique provides insights into the spatiotemporal photoionization dynamics of molecular shape resonances. We find delays of up to ∼600 as in the ionization out of the highest occupied molecular orbital (HOMO) with a strong dependence on the emission direction and a pronounced asymmetry along the dissociation axis. Comparison with quantum-scattering calculations traces the asymmetries to the interference of a small subset of partial waves at low kinetic energies and, additionally, to the interference of two overlapping shape resonances in the HOMO-1 channel. Our experimental and theoretical results establish a broadly applicable approach to space- and time-resolved photoionization dynamics in the molecular frame.

Photoelectron Spectroscopy of Liquid Water with Tunable Extreme-Ultraviolet Radiation: Effects of Electron Scattering.

We report the first systematic photoelectron measurements of the three outer-valence bands of liquid water as a function of the ionizing photon energy in the near-threshold region. We use extreme-ultraviolet (XUV) radiation tunable between ∼17.1 and 35.6 eV, obtained through monochromatization of a high-harmonic source. We show that the absolute values of the apparent vertical ionization energies and their respective peak widths show a decreasing trend of their magnitudes with increasing photon energy close to the ionization threshold. We find that the observed effects do not only depend on the electron kinetic energy but are also different for the various outer-valence bands. These observations are consistent with, but not fully explained by, the effects of inelastic electron scattering.

Attosecond spectroscopy of liquid water.

Electronic dynamics in liquids are of fundamental importance, but time-resolved experiments have so far remained limited to the femtosecond time scale. We report the extension of attosecond spectroscopy to the liquid phase. We measured time delays of 50 to 70 attoseconds between the photoemission from liquid water and that from gaseous water at photon energies of 21.7 to 31.0 electron volts. These photoemission delays can be decomposed into a photoionization delay sensitive to the local environment and a delay originating from electron transport. In our experiments, the latter contribution is shown to be negligible. By referencing liquid water to gaseous water, we isolated the effect of solvation on the attosecond photoionization dynamics of water molecules. Our methods define an approach to separating bound and unbound electron dynamics from the structural response of the solvent.

Ionization Energy of Liquid Water Revisited.

The ionization energy of liquid water is one of its most fundamental properties, an important benchmark for first-principles electronic-structure calculations and a crucial reference in the growing field of liquid-phase photoelectron spectroscopy. Despite this significance, a consensus on its value appears to be missing in the literature. Therefore, we use a monochromatized high-harmonic light source to perform detailed measurements of the ionization energy of liquid water in the presence of a tunable bias voltage applied to the liquid jet. Our results suggest that this simple method is sufficient to simultaneously compensate the effects of the streaming potential and that of the vacuum-level offset between the liquid and the photoelectron spectrometer. Our measurements yield corrected values of the vertical and adiabatic ionization energies of the 1b1 band of bulk liquid water of 11.67(15) and 10.12(15) eV, respectively. Our method is broadly applicable and is likely to result in corrections to the measured ionization energies of solvated species as well.

Alternative Approach for the Determination of Mean Free Paths of Electron Scattering in Liquid Water Based on Experimental Data.

Mean free paths of low-energy electrons in liquid water are of importance for modeling many physicochemical processes, but neither theoretical predictions nor experimental results have converged for these parameters. We therefore introduce an approach to determine elastic and inelastic mean free paths (EMFP, IMFP) based on experimental data. We show that ab initio calculations of electron scattering with water clusters converge with cluster size, thus providing access to condensed-phase scattering. The results are used in Monte Carlo simulations to extract EMFP and IMFP from recent liquid-microjet experiments that determined the effective attenuation length (EAL) and the photoelectron angular distribution (PAD) following oxygen 1s-ionization of liquid water. For electron kinetic energies from 10 to 300 eV, we find that the IMFP is noticeably larger than the EAL. The EMFP is longer than that of gas-phase water and the IMFP is longer compared to latest theoretical estimations, but both EMFP and IMFP are much shorter than suggested by experimental measurements of integral cross sections for amorphous ice.