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.

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We report measurements of energy-dependent attosecond photoionization delays between the two outer-most valence shells of N2O and H2O. The combination of single-shot signal referencing with the use of different metal foils to filter the attosecond pulse train enables us to extract delays from congested spectra. Remarkably large delays up to 160 as are observed in N2O, whereas the delays in H2O are all smaller than 50 as in the photon-energy range of 20-40 eV. These results are interpreted by developing a theory of molecular photoionization delays. The long delays measured in N2O are shown to reflect the population of molecular shape resonances that trap the photoelectron for a duration of up to ∼110 as. The unstructured continua of H2O result in much smaller delays at the same photon energies. Our experimental and theoretical methods make the study of molecular attosecond photoionization dynamics accessible.Photoionization and photoelectron spectroscopies are powerful approaches to measuring the electronic structure of matter [1,2]. A complete quantum-mechanical description of photoionization, both in the time and frequency domains, requires the amplitude and phase of all dipole matrix elements, e.g. in a partial-wave expansion. Most experiments to date measure photoionization cross sections which are described by the sum of squared moduli of individual partial-wave dipole matrix elements. Cross sections thus contain no information about the partial-wave phase shifts. In contrast, photoelectron angular distributions are highly sensitive to partial-wave phase shifts [3,4] between continua associated with the same ionic state. Phase-shifts between continua associated with different ionization thresholds are not measurable in the frequency domain because the lack of spectral overlap between the corresponding photoelectrons erases the coherence required to measure such phase shifts.In this letter, we show that attosecond metrology can be employed to measure this information in molecular photoionization. Specifically, we study the effect of molecular shape resonances on the measured photoionization delays. When the combined molecular (or atomic) and centrifugal potential felt by the photoelectron displays a barrier, one or several quasi-bound states can emerge [5][6][7]. These resonances decay by tunneling through the potential barrier and often lead to a local enhancement of the photoionization cross section. Such shape resonances have so far only been measured by frequency-resolved measurements. Here, we show that attosecond metrology provides access to the time-domain manifestation of shape resonances. In the case of N 2 O, our measurements indeed reveal surprisingly large delays reaching up to 160 as in the range of 20 to 40 eV. In contrast, delays measured at the same photon energies in H 2 O all lie below 50 as in magnitude. These results are interpreted by developing a theory of molecular photoionization delays relying on accurate molecular scattering calculations. This analysis shows that the delays measured ...

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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.