Hans Jakob Wörner

Bio: Hans Jakob Wörner is an academic researcher from ETH Zurich. The author has contributed to research in topics: Attosecond & Ionization. The author has an hindex of 39, co-authored 177 publications receiving 5455 citations. Previous affiliations of Hans Jakob Wörner include National Research Council & University of Ottawa.

Measurement and laser control of attosecond charge migration in ionized iodoacetylene

Abstract: The ultrafast motion of electrons and holes after light-matter interaction is fundamental to a broad range of chemical and biophysical processes. We advanced high-harmonic spectroscopy to resolve spatially and temporally the migration of an electron hole immediately after ionization of iodoacetylene while simultaneously demonstrating extensive control over the process. A multidimensional approach, based on the measurement and accurate theoretical description of both even and odd harmonic orders, enabled us to reconstruct both quantum amplitudes and phases of the electronic states with a resolution of ~100 attoseconds. We separately reconstructed quasi-field-free and laser-controlled charge migration as a function of the spatial orientation of the molecule and determined the shape of the hole created by ionization. Our technique opens the prospect of laser control over electronic primary processes.

Following a chemical reaction using high-harmonic interferometry

Abstract: New methods are emerging that aim to image chemical reactions as they occur, using X-ray diffraction, electron diffraction or laser-induced recollision. But none of these methods offer spectral selection, which allows a laser pulse with light of one wavelength to initiate a reaction, and a second pulse with another, appropriately selected wavelength to monitor the reacting molecules. Worner et al. now show that this apparent limitation offers exciting opportunities for recollision-based high-harmonic spectroscopy: due to the coherent nature of the attosecond high-harmonic pulse generation, unexcited molecules can act as local oscillators against which structural and electronic dynamics is observed on an attosecond timescale. High-harmonic spectroscopy thus seems ideally suited to measure coupled electronic and nuclear dynamics in fast photochemical reactions, or to characterize short-lived transition states. New methods are emerging that aim to image chemical reactions as they occur using X-ray diffraction, electron diffraction or laser-induced recollision, but spectral selection cannot be used to monitor the reacting molecules for these methods. These authors show that this apparent limitation offers opportunities for recollision-based high-harmonic spectroscopy, in which unexcited molecules can act as local oscillators against which structural and electronic dynamics is observed on an attosecond timescale. The study of chemical reactions on the molecular (femtosecond) timescale typically uses pump laser pulses to excite molecules and subsequent probe pulses to interrogate them. The ultrashort pump pulse can excite only a small fraction of molecules, and the probe wavelength must be carefully chosen to discriminate between excited and unexcited molecules. The past decade has seen the emergence of new methods that are also aimed at imaging chemical reactions as they occur, based on X-ray diffraction1, electron diffraction2 or laser-induced recollision3,4—with spectral selection not available for any of these new methods. Here we show that in the case of high-harmonic spectroscopy based on recollision, this apparent limitation becomes a major advantage owing to the coherent nature of the attosecond high-harmonic pulse generation. The coherence allows the unexcited molecules to act as local oscillators against which the dynamics are observed, so a transient grating technique5,6 can be used to reconstruct the amplitude and phase of emission from the excited molecules. We then extract structural information from the amplitude, which encodes the internuclear separation, by quantum interference at short times and by scattering of the recollision electron at longer times. The phase records the attosecond dynamics of the electrons, giving access to the evolving ionization potentials and the electronic structure of the transient molecule. In our experiment, we are able to document a temporal shift of the high-harmonic field of less than an attosecond (1 as = 10−18 s) between the stretched and compressed geometry of weakly vibrationally excited Br2 in the electronic ground state. The ability to probe structural and electronic features, combined with high time resolution, make high-harmonic spectroscopy ideally suited to measuring coupled electronic and nuclear dynamics occurring in photochemical reactions and to characterizing the electronic structure of transition states.

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

Abstract: 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.

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

Abstract: 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 CF 4 + and SF 6 + 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.

Probing collective multi-electron dynamics in xenon with high-harmonic spectroscopy

Abstract: High-harmonic spectroscopy probes atomic structure by looking at the short-wavelength emission excited from atoms by ultrafast pulses of laser light. It is now shown that this technique can even detect signatures of electron–electron interactions.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Molecular dynamics simulations and drug discovery

Abstract: This review discusses the many roles atomistic computer simulations of macromolecular (for example, protein) receptors and their associated small-molecule ligands can play in drug discovery, including the identification of cryptic or allosteric binding sites, the enhancement of traditional virtual-screening methodologies, and the direct prediction of small-molecule binding energies. The limitations of current simulation methodologies, including the high computational costs and approximations of molecular forces required, are also discussed. With constant improvements in both computer power and algorithm design, the future of computer-aided drug design is promising; molecular dynamics simulations are likely to play an increasingly important role.

X-Rays and Extreme Ultraviolet Radiation: Principles and Applications

Abstract: This self-contained, comprehensive book describes the fundamental properties of soft x-rays and extreme ultraviolet (EUV) radiation and discusses their applications in a wide variety of fields, including EUV lithography for semiconductor chip manufacture and soft x-ray biomicroscopy. The author begins by presenting the relevant basic principles such as radiation and scattering, wave propagation, diffraction, and coherence. He then goes on to examine a broad range of phenomena and applications. The topics covered include EUV lithography, biomicroscopy, spectromicroscopy, EUV astronomy, synchrotron radiation, and soft x-ray lasers. He also provides a great deal of useful reference material such as electron binding energies, characteristic emission lines and photo-absorption cross-sections. The book will be of great interest to graduate students and researchers in engineering, physics, chemistry, and the life sciences. It will also appeal to practicing engineers involved in semiconductor fabrication and materials science.