Martin Huppert

Time-resolved X-ray absorption spectroscopy with a water-window high-harmonic source

An x-ray view of C–F and S–F bond breaks X-ray absorption spectroscopy is a useful probe of element-specific dynamics in molecular reactions. However, the required x-ray fluxes have rarely been available outside expensive dedicated facilities such as synchrotrons. Pertot et al. developed a tabletop laser-based high-harmonic source that extends far enough into the x-ray region to probe carbon K-edge and sulfur L-edge absorptions with femtosecond temporal resolution. They used this source to track the previously elusive dissociative dynamics of gaseous carbon tetrafluoride and sulfur hexafluoride after laser-induced ionization. Science, this issue p. 264 Ultrafast x-ray absorption spectroscopy at carbon and sulfur frequencies tracks dissociative dynamics of CF4+ and SF6+. Time-resolved x-ray absorption spectroscopy (TR-XAS) has so far practically been limited to large-scale facilities, to subpicosecond temporal resolution, and to the condensed phase. We report the realization of TR-XAS with a temporal resolution in the low femtosecond range by developing a tabletop high-harmonic source reaching up to 350 electron volts, thus partially covering the spectral region of 280 to 530 electron volts, where water is transmissive. We used this source to follow previously unexamined light-induced chemical reactions in the lowest electronic states of isolated CF4+ and SF6+ molecules in the gas phase. By probing element-specific core-to-valence transitions at the carbon K-edge or the sulfur L-edges, we characterized their reaction paths and observed the effect of symmetry breaking through the splitting of absorption bands and Rydberg-valence mixing induced by the geometry changes.

Streaking of 43-attosecond soft-X-ray pulses generated by a passively CEP-stable mid-infrared driver

Attosecond metrology has so far largely remained limited to titanium:sapphire lasers combined with an active stabilization of the carrier-envelope phase (CEP). These sources limit the achievable photon energy to ∼100 eV which is too low to access X-ray absorption edges of most second- and third-row elements which are central to chemistry, biology and material science. Therefore, intense efforts are underway to extend attosecond metrology to the soft-X-ray (SXR) domain using mid-infrared (mid-IR) drivers. Here, we introduce and experimentally demonstrate a method that solves the long-standing problem of the complete temporal characterization of ultra-broadband (≫10 eV) attosecond pulses. We generalize the recently proposed Volkov-transform generalized projection algorithm (VTGPA) to the case of multiple overlapping photoelectron spectra and demonstrate its application to isolated attosecond pulses. This new approach overcomes all key limitations of previous attosecond-pulse reconstruction methods, in particular the central-momentum approximation (CMA), and it incorporates the physical, complex-valued and energy-dependent photoionization matrix elements. These properties make our approach general and particularly suitable for attosecond supercontinua of arbitrary bandwidth. We apply this method to attosecond SXR pulses generated from a two-cycle mid-IR driver, covering a bandwidth of ∼100 eV and reaching photon energies up to 180 eV. We extract an SXR pulse duration of (43±1) as from our streaking measurements, defining a new world record. Our results prove that the popular and broadly available scheme of post-compressing the output of white-light-seeded optical parametric amplifiers is adequate to produce high-contrast isolated attosecond pulses covering the L-edges of silicon, phosphorous and sulfur. Our new reconstruction method and experimental results open the path to the production and characterization of attosecond pulses lasting less than one atomic unit of time (24 as) and covering X-ray absorption edges of most light elements.

Photoelectron spectrometer for attosecond spectroscopy of liquids and gases

A new apparatus for attosecond time-resolved photoelectron spectroscopy of liquids and gases is described. It combines a liquid microjet source with a magnetic-bottle photoelectron spectrometer and an actively stabilized attosecond beamline. The photoelectron spectrometer permits venting and pumping of the interaction chamber without affecting the low pressure in the flight tube. This pressure separation has been realized through a sliding skimmer plate, which effectively seals the flight tube in its closed position and functions as a differential pumping stage in its open position. A high-harmonic photon spectrometer, attached to the photoelectron spectrometer, exit port is used to acquire photon spectra for calibration purposes. Attosecond pulse trains have been used to record photoelectron spectra of noble gases, water in the gas and liquid states as well as solvated species. RABBIT scans demonstrate the attosecond resolution of this setup.

Attosecond beamline with actively stabilized and spatially separated beam paths

We describe a versatile and compact beamline for attosecond spectroscopy. The setup consists of a high-order harmonic source followed by a delay line that spatially separates and then recombines the extreme-ultraviolet (XUV) and residual infrared (IR) pulses. The beamline introduces a controlled and actively stabilized delay between the XUV and IR pulses on the attosecond time scale. A new active-stabilization scheme combining a helium-neon-laser and a white-light interferometer minimizes fluctuations and allows to control delays accurately (26 as rms during 1.5 h) over long time scales. The high-order-harmonic-generation region is imaged via optical systems, independently for XUV and IR, into an interaction volume to perform pump-probe experiments. As a consequence of the spatial separation, the pulses can be independently manipulated in intensity, polarization, and frequency content. The beamline can be combined with a variety of detectors for measuring attosecond dynamics in gases, liquids, and solids.

A table-top monochromator for tunable femtosecond XUV pulses generated in a semi-infinite gas cell: Experiment and simulations

We present a new design of a time-preserving extreme-ultraviolet (XUV) monochromator using a semi-infinite gas cell as a source. The performance of this beamline in the photon-energy range of 20 eV-42 eV has been characterized. We have measured the order-dependent XUV pulse durations as well as the flux and the spectral contrast. XUV pulse durations of ≤40 fs using 32 fs, 800 nm driving pulses were measured on the target. The spectral contrast was better than 100 over the entire energy range. A simple model based on the strong-field approximation is presented to estimate different contributions to the measured XUV pulse duration. On-axis phase-matching calculations are used to rationalize the variation of the photon flux with pressure and intensity.

In situ frequency gating and beam splitting of vacuum- and extreme-ultraviolet pulses

Monochromatization of high-harmonic sources has opened fascinating perspectives regarding time-resolved photoemission from all phases of matter. Such studies have invariably involved the use of spectral filters or spectrally dispersive optical components that are inherently lossy and technically complex. Here we present a new technique for the spectral selection of near-threshold harmonics and their spatial separation from the driving beams without any optical elements. We discover the existence of a narrow phase-matching gate resulting from the combination of the non-collinear generation geometry in an extended medium, atomic resonances and absorption. Our technique offers a filter contrast of up to 104 for the selected harmonics against the adjacent ones and offers multiple temporally synchronized beamlets in a single unified scheme. We demonstrate the selective generation of 133, 80 or 56 nm femtosecond pulses from a 400-nm driver, which is specific to the target gas. These results open new pathways towards phase-sensitive multi-pulse spectroscopy in the vacuum- and extreme-ultraviolet, and frequency-selective output coupling from enhancement cavities.

Relativistic photoionization delays and the role of auto-ionizing resonances

We measured the relative ionization delays into the two spin-orbit components of the electronic ground states of Xe+ and Kr+ under the inuence of auto-ionizing intermediate and final states. The results are in good agreement with state-of-the-art calculations based on the time-dependent configuration-interaction singles (TDCIS) approach.

Attosecond time-resolved photoelectron spectroscopy of liquids

Attosecond spectroscopy has offered promising insights into ultrafast dynamics in solid and gas-phase systems, but has not been extended to the liquid phase yet. We report the achievement of this fundamental advance and demonstrate the measurement of attosecond photoionization delays from liquid water. In combination with theory, these delays reveal electron propagation effects through the aqueous environment and dephasing processes. These results are expected to provide new insights into the nature of photoionization and electron transport in liquid water.

Attosecond streaking of soft-X-ray pulses generated by a mid-IR laser

Attosecond metrology has so far remained limited to Ti:Sa lasers combined with an active stabilization of the carrier-envelope phase (CEP), where the achievable photon energy is limited to ∼100eV. This is too low to access X-ray absorption edges of most second- and third-row elements which are central to chemistry, biology and material science. The quadratic scaling of the ponderomotive energy with wavelength enables to generate photon energies in the soft-X-ray (SXR) domain with mid-IR laser sources. We use our commercial TiSa laser system (5mJ, 1kHz) to generate 1.8μm mid-IR pulses with an OPA (TOPAS HE, conversion efficiency 34%), which is spectrally broadened to an octave in a static filled hollow core fiber delivering pulse energies of up to 540 μJ. Recompression to the few-cycle regime is performed by a pair of fused-silica wedges. A fraction of the beam is used to measure the CEP stability with a single-shot stereo-ATI phase meter. The CEP stability was found to be better than 300 mrad rms, remarkably better than most previously reported values [1].

Ultrafast time-resolved photoelectron spectroscopy of solvated systems

Photoionization delays reveal electron dynamics on ultrashort time scales. Here, we present attosecond time-resolved experimental results of gas-phase and liquid water and we discuss the analysis methods to retrieve molecular ionization delays therefrom.