Femtosecond proton transfer in urea solutions probed by X-ray spectroscopy.

Proton transfer is one of the most fundamental events in aqueous-phase chemistry and an emblematic case of coupled ultrafast electronic and structural dynamics1,2. Disentangling electronic and nuclear dynamics on the femtosecond timescales remains a formidable challenge, especially in the liquid phase, the natural environment of biochemical processes. Here we exploit the unique features of table-top water-window X-ray absorption spectroscopy3-6 to reveal femtosecond proton-transfer dynamics in ionized urea dimers in aqueous solution. Harnessing the element specificity and the site selectivity of X-ray absorption spectroscopy with the aid of ab initio quantum-mechanical and molecular-mechanics calculations, we show how, in addition to the proton transfer, the subsequent rearrangement of the urea dimer and the associated change of the electronic structure can be identified with site selectivity. These results establish the considerable potential of flat-jet, table-top X-ray absorption spectroscopy7,8 in elucidating solution-phase ultrafast dynamics in biomolecular systems.

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.

Time-Resolved Multielectron Coincidence Spectroscopy of Double Auger-Meitner Decay Following Xe 4d Ionization.

We introduce time-resolved multielectron coincidence spectroscopy and apply it to the double Auger-Meitner (AM) emission process following xenon 4d photoionization. The photoelectron and AM electron(s) are measured in coincidence by using a magnetic-bottle time-of-flight spectrometer, enabling an unambiguous assignment of the complete cascade pathways involving two AM electron emissions. In the presence of a near-infrared (NIR) laser pulse, the intermediate Xe^{2+*} state embedded in the Xe^{3+} continuum is probed through single NIR photon absorption and the lifetime of this intermediate Xe^{2+*} state is directly obtained as (109±22) fs.

Ultrafast Imaging of the Jahn-Teller Topography in Carbon Tetrachloride.

We report the direct time-domain observation of ultrafast dynamics driven by the Jahn-Teller effect. Using time-resolved photoelectron spectroscopy with a vacuum-ultraviolet femtosecond source to prepare high-lying Rydberg states of carbon tetrachloride, our measurements reveal the local topography of a Jahn-Teller conical intersection. The pump pulse prepares a configurationally mixed superposition of the degenerate 1T2 4p-Rydberg states, which then distorts through spontaneous symmetry breaking that we identify to follow the t2 bending motion. Photoionization of these states to three cationic states 2T1, 2T2, and 2E reveals a shift in the center-of-mass of the photoelectron peaks associated with the 2Tn states which reveals the local topography of the Jahn-Teller conical intersection region prepared by the pump pulse. Time-dependent density functional theory calculations confirm that the dominant nuclear motion observed in the spectrum is the CCl4 t2 bending mode. The large density of states in the VUV spectral region at 9.33 eV of carbon tetrachloride and strong vibronic coupling result in ultrafast decay of the excited-state signal with a time constant of 75(4) fs.

High-harmonic generation in liquids with few-cycle pulses: effect of laser-pulse duration on the cut-off energy.

High-harmonic generation (HHG) in liquids is opening new opportunities for attosecond light sources and attosecond time-resolved studies of dynamics in the liquid phase. In gas-phase HHG, few-cycle pulses are routinely used to create isolated attosecond pulses and to extend the cut-off energy. Here, we study the properties of HHG in liquids, including heavy water, ethanol and isopropanol, by continuously tuning the pulse duration of a mid-infrared driver from the multi- to the two-cycle regime. Similar to the gas phase, we observe the transition from discrete odd-order harmonics to continuous extreme-ultraviolet emission. However, the cut-off energy is shown to be entirely independent of the pulse duration. These observations are confirmed by ab-initio simulations of HHG in large liquid clusters. Our results support the notion that the cut-off energy is a fundamental property of the liquid, independent of the driving-pulse properties. Our work implies that few-cycle mid-infrared laser pulses are suitable drivers for generating isolated attosecond pulses from liquids and confirm the capability of high-harmonic spectroscopy to determine the mean-free paths of slow electrons in liquids.

Observation of Nuclear Wave-Packet Interference in Ultrafast Interatomic Energy Transfer.

We report the experimental observation of quantum interference in the nuclear wave-packet dynamics driving ultrafast excitation-energy transfer in argon dimers below the threshold of interatomic Coulombic decay (ICD). Using time-resolved photoion-photoion coincidence spectroscopy and quantum dynamics simulations, we reveal that the electronic relaxation dynamics of the inner-valence 3s hole on one atom leading to a 4s or 4p excitation on the other one is influenced by nuclear quantum dynamics in the initial state, giving rise to a deep, periodic modulation on the kinetic-energy-release (KER) spectra of the coincident Ar^{+}-Ar^{+} ion pairs. Moreover, the time-resolved KER spectra show characteristic fingerprints of quantum interference effects during the energy-transfer process. Our findings pave the way to elucidating quantum-interference effects in ultrafast charge- and energy-transfer dynamics in more complex systems, such as molecular clusters and solvated molecules.

Effects of Autoionizing Resonances on Wave-Packet Dynamics Studied by Time-Resolved Photoelectron Spectroscopy.

We report a combined experimental and theoretical study on the effect of autoionizing resonances in time-resolved photoelectron spectroscopy. The coherent excitation of N_{2} by ∼14.15 eV extreme-ultraviolet photons prepares a superposition of three dominant adjacent vibrational levels (v^{'}=14-16) in the valence b^{'} ^{1}Σ_{u}^{+} state, which are probed by the absorption of two or three near-infrared photons (800 nm). The superposition manifests itself as coherent oscillations in the measured photoelectron spectra. A quantum-mechanical simulation confirms that two autoionizing Rydberg states converging to the excited A ^{2}Π_{u} and B ^{2}Σ_{u}^{+} N_{2}^{+} cores are accessed by the resonant absorption of near-infrared photons. We show that these resonances apply different filters to the observation of the vibrational wave packet, which results in different phases and amplitudes of the oscillating photoelectron signal depending on the nature of the autoionizing resonance. This work clarifies the importance of resonances in time-resolved photoelectron spectroscopy and particularly reveals the phase of vibrational quantum beats as a powerful observable for characterizing the properties of such resonances.

Generation and complete polarimetry of ultrashort circularly polarized extreme-ultraviolet pulses.

The generation of ultrashort circularly polarized pulses in the extreme-ultraviolet spectral range has recently attracted considerable interest for applications in time-resolved circular-dichroism experiments. Here, we demonstrate a simple approach to generate near-circularly polarized femtosecond pulses in the vacuum-ultraviolet. The ellipticity of the generated light can be continuously tuned from linear to near-circular, as demonstrated by detailed polarimetry measurements. Combining optical polarimetry with photoelectron circular-dichroism (PECD) measurements, we demonstrate a novel approach to characterizing the polarization state of light in terms of all four Stokes parameters. For photon energies of 9.3 eV, we obtained S3 = 0.96 ± 0.02 and a degree of polarization of 97±2%, i.e. the highest values reported from any harmonic-generation source so far. This source is directly applicable to circular-dichroism experiments, also enabling time-resolved PECD in the extreme-ultraviolet, a general approach to probing time-dependent chirality during chemical processes on (sub)-femtosecond time scales.

Energy scaling of carrier-envelope-phase-stable sub-two-cycle pulses at 1.76 µm from hollow-core-fiber compression to 1.9 mJ.

We present the energy scaling of a sub-two-cycle (10.4 fs) carrier-envelope-phase-stable light source centered at 1.76 µm to 1.9 mJ pulse energy. The light source is based on an optimized spectral-broadening scheme in a hollow-core fiber and a consecutive pulse compression with bulk material. This is, to our knowledge, the highest pulse energy reported to date from this type of sources. We demonstrate the application of this improved source to the generation of bright water-window soft-X-ray high harmonics. Combined with the short pulse duration, this source paves the way to the attosecond time-resolved water-window spectroscopy of complex molecules in aqueous solutions.

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.

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.

Ground-State Photoelectron Circular Dichroism of Methyl p-Tolyl Sulfoxide by Single-Photon Ionisation from a Table-Top Source.

Single-photon ionisation of enantiopure methyl p-tolyl sulfoxide by circularly polarised light at 133 nm shows remarkably strong photoelectron circular dichroism (PECD), which has been measured in a velocity-map-imaging spectrometer. Both enantiomers were measured, each showing a PECD of a similar magnitude (ca. 25 %). These experiments were carried out with a tabletop high-harmonic source with a photon energy of 9.3 eV, capable of ionising the electronic ground state of most organic and inorganic molecules. Ab-initio scattering calculations provide a theoretical value of the expected chiral asymmetry parameter, and agree very well with the measured values once orbital mixing via configuration interaction in the cation is taken into account. This study demonstrates a simple photoionisation scheme that can be readily applied to study the time-resolved PECD of photochemical reactions and suggests a pronounced sensitivity of PECD to electronic configuration interaction in the cation.

The ultrafast vibronic dynamics of ammonia's D̃ state.

Using vacuum-ultraviolet time-resolved velocity map imaging of photoelectrons, we study ultrafast coupled electronic and nuclear dynamics in low-lying Rydberg states of ammonia. Vibrationally-resolved internal vibrational relaxation (IVR) is observed in a progression of the e' bending modes. This vibrational progression is only observed in the D̃ state, and is lost upon ultrafast internal conversion to the C̃ and B̃ electronic states. Due to the ultrashort time scale of the internal conversion (ca. 64 fs), and the vibronic resolution, the non-adiabatic coupling vectors are identified and verified with ab initio calculations. The time-scale of this IVR process is highly surprising and significant because IVR is usually treated as an incoherent process that proceeds statistically, according to a "Fermi's Golden Rule"-like model, where the process scales with the available degrees of freedom. Here, we show that it can be highly non-statistical, restricted to only a very small subset of vibrational motions.

Real-time probing of chirality during a chemical reaction.

Chiral molecules interact and react differently with other chiral objects, depending on their handedness. Therefore, it is essential to understand and ultimately control the evolution of molecular chirality during chemical reactions. Although highly sophisticated techniques for the controlled synthesis of chiral molecules have been developed, the observation of chirality on the natural femtosecond time scale of a chemical reaction has so far remained out of reach in the gas phase. Here, we demonstrate a general experimental technique, based on high-harmonic generation in tailored laser fields, and apply it to probe the time evolution of molecular chirality during the photodissociation of 2-iodobutane. These measurements show a change in sign and a pronounced increase in the magnitude of the chiral response over the first 100 fs, followed by its decay within less than 500 fs, revealing the photodissociation to achiral products. The observed time evolution is explained in terms of the variation of the electric and magnetic transition-dipole moments between the lowest electronic states of the cation as a function of the reaction coordinate. These results open the path to investigations of the chirality of molecular-reaction pathways, light-induced chirality in chemical processes, and the control of molecular chirality through tailored laser pulses.

Attosecond Photoionization Dynamics: from Molecules over Clusters to the Liquid Phase.

Photoionization is a process taking place on attosecond time scales. How its properties evolve from isolated particles to the condensed phase is an open question of both fundamental and practical relevance. Here, we review recent work that has advanced the study of photoionization dynamics from atoms to molecules, clusters and the liquid phase. The first measurements of molecular photoionization delays have revealed the attosecond dynamics of electron emission from a molecular shape resonance and their sensitivity to the molecular potential. Using electron-ion coincidence spectroscopy these measurements have been extended from isolated molecules to clusters. A continuous increase of the delays with the water-cluster size has been observed up to a size of 4-5 molecules, followed by a saturation towards larger clusters. Comparison with calculations has revealed a correlation of the time delay with the spatial extension of the created electron hole. Using cylindrical liquid-microjet techniques, these measurements have also been extended to liquid water, revealing a delay relative to isolated water molecules that was very similar to the largest water clusters studied. Detailed modeling based on Monte-Carlo simulations confirmed that these delays are dominated by the contributions of the first two solvation shells, which agrees with the results of the cluster measurements. These combined results open the perspective of experimentally characterizing the delocalization of electronic wave functions in complex systems and studying their evolution on attosecond time scales.

Polarization measurements of deep- to extreme-ultraviolet high harmonics generated in liquid flat sheets.

Laboratory-based coherent light sources enable a wide range of applications to investigate dynamical processes in matter. High-harmonic generation (HHG) from liquid samples is a recently discovered coherent source of extreme-ultraviolet (XUV) radiation potentially capable of achieving few-femtosecond to attosecond pulse durations. However, the polarization state of this light source has so far remained unknown. In this work, we characterize the degree of polarization of both low- and high-order harmonics generated from liquid samples using linearly polarized 400 nm and 800 nm drivers. We find a remarkably high degree of linear polarization of harmonics ranging all the way from the deep-ultraviolet (160 nm) across the vacuum-ultraviolet into the XUV domain (73 nm). These results establish high-harmonic generation in liquids as a promising alternative to conventional sources of XUV radiation, combining the benefits of high target densities comparable to solids with a continuous sample renewal that avoids the limitations imposed by laser-induced damage.

Quantitative uncertainty determination of phase retrieval in RABBITT.

Reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) is one of the most widely used approaches to measure the time delays in photoionization. The time delay, which corresponds to a phase difference of two oscillating signals, is usually retrieved by cosine fitting or fast Fourier transform (FFT). We propose two estimators for the phase uncertainty of cosine fitting from the signal per se of an individual experiment: (i) σ(φ fit)≈B A2N, where B/A is the mean-value-to-amplitude ratio, and N is the total count number, and (ii) σ(φ fit)≈1-R 2 R 2 n bins, where nbins is the total number of bins in the time domain, and R2 is the coefficient of determination. The former estimator is applicable for the statistical fluctuation, while the latter includes the effects from various uncertainty sources, which is mathematically proven and numerically validated. This leads to an efficient and reliable approach to determining quantitative uncertainties in RABBITT experiments and evaluating the observed discrepancy among individual measurements, as demonstrated on the basis of experimental data.

Time-resolved formation of excited atomic and molecular states in XUV-induced nanoplasmas in ammonia clusters.

High intensity XUV radiation from a free-electron laser (FEL) was used to create a nanoplasma inside ammonia clusters with the intent of studying the resulting electron-ion interactions and their interplay with plasma evolution. In a plasma-like state, electrons with kinetic energy lower than the local collective Coulomb potential of the positive ionic core are trapped in the cluster and take part in secondary processes (e.g. electron-impact excitation/ionization and electron-ion recombination) which lead to subsequent excited and neutral molecular fragmentation. Using a time-delayed UV laser, the dynamics of the excited atomic and molecular states are probed from -0.1 ps to 18 ps. We identify three different phases of molecular fragmentation that are clearly distinguished by the effect of the probe laser on the ionic and electronic yield. We propose a simple model to rationalize our data and further identify two separate channels leading to the formation of excited hydrogen.

Electronic and vibrational relaxation dynamics of NH3 Rydberg states probed by vacuum-ultraviolet time-resolved photoelectron imaging.

Time-resolved dynamics of high-lying Rydberg states of ammonia (NH3) prepared by using a vacuum ultraviolet (VUV) pump (∼9.3 eV) and an ultraviolet (UV) probe (∼4.7 eV) pulse are reported using photoelectron imaging detection. After photoexcitation, two main features appear in the photoelectron spectrum with vertical binding energies of ∼1.8 eV and ∼3.2 eV and with distinctly different anisotropy parameters ß of ∼1.3 and ∼0.7, respectively. This information allows the unambiguous assignment of the respective Rydberg states and disentangles the induced electronic and vibrational dynamics. The combination of velocity-map imaging with femtosecond VUV and UV pulses is shown to offer an attractive approach for studying the dynamics of high-lying Rydberg states of small molecules.

Complete reconstruction of ultra-broadband isolated attosecond pulses including partial averaging over the angular distribution.

Attosecond streaking is a powerful tool to investigate ultrafast electron dynamics on the attosecond time scale. To obtain the highest temporal resolution in a pump-probe experiment, soft-X-ray (SXR) and infrared (IR) pulses have to be carefully characterized. Here, we present a detailed description of our recent generalization of the Volkov-transform generalized projection algorithm (VTGPA) and its application to multiple overlapping photoelectron bands. This method allows for the complete temporal reconstruction of both IR and SXR pulses under the inclusion of accurate complex photoionization matrix elements (PMEs). In this article, we compare the performance of our new method with traditional algorithms. We particularly focus on the important role played by the photoelectron angular distribution (PAD) which needs to be taken into account for the highest fidelity of attosecond pulse reconstruction. For this purpose, we investigate numerically the influence of the finite collection angle of the electron spectrometer on the retrieval and the obtained pulse parameters. We further theoretically demonstrate the reliability of the reconstruction for pulse durations even shorter than the atomic unit of time.