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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Conical intersections allow electronically excited molecules to return to their electronic ground state. Here, we observe the fastest electronic relaxation dynamics measured to date by extending attosecond transient-absorption spectroscopy (ATAS) to the carbon K-edge. We selectively launch wave packets in the two lowest electronic states (D0 and D1) of C2H4+. The electronic D1 → D0 relaxation takes place with a short time constant of 6.8 ± 0.2 femtoseconds. The electronic-state switching is directly visualized in ATAS owing to a spectral separation of the D1 and D0 bands caused by electron correlation. Multidimensional structural dynamics of the molecule are simultaneously observed. Our results demonstrate the capability to resolve the fastest electronic and structural dynamics in the broad class of organic molecules. They show that electronic relaxation in the prototypical organic chromophore can take place within less than a single vibrational period.

Recent advances in high-harmonic generation gave rise to soft X-ray pulses with higher intensity, shorter duration and higher photon energy. One of the remaining shortages of this source is its restriction to linear polarization, since the yield of generation of elliptically polarized high harmonics has been low so far. We here show how this limitation is overcome by using a cross-polarized two-colour laser field. With this simple technique, we reach high degrees of ellipticity (up to 75%) with efficiencies similar to classically generated linearly polarized harmonics. To demonstrate these features and to prove the capacity of our source for applications, we measure the X-ray magnetic circular dichroism (XMCD) effect of nickel at the M 2,3 absorption edge around 67 eV. There results open up the way towards femtosecond time-resolved experiments using high harmonics exploiting the powerful element-sensitive XMCD effect and resolving the ultrafast magnetization dynamics of individual components in complex materials.