At a most fundamental level, electronic motion determines all aspect of technology and life. Electrons determine whether and how a molecule dissociates and how a chemical reaction occurs; electrons sustain the energy flow in human vision or of our nerve system, and they determine whether biological life ends. Our aim is to zoom into the microcosm of electronic motion to observe and possibly control their dynamics. The research therefore deals with a wide range of aspects from the quantum nature of electronic correlation to tracking their dynamics on the attosecond timescale despite velocities thousand times faster than that of the nuclei. We have, e.g. pioneered attosecond angular streaking, which permits attosecond resolution measurements of electron wavepackets without attosecond x-ray pulses. Currently, we use a reaction microscope to investigate the concerted motion of electrons and ions on their attosecond timescale and to realize self-imaging of molecular structure.
Quantum Dynamics
OURCONTRIBUTIONS
Angular Streaking "Attoclock"
Ultrashort measurement-time resolution is traditionally obtained in pump–probe experiments, for which two ultrashort light pulses are required; the time resolution is then determined by the pulse duration. But although pulses of subfemtosecond duration are available, so far the energy of these pulses is too low to fully implement the traditional pump–probe technique. Here, we demonstrate attosecond angular as an alternative approach to achieve attosecond time resolution. The method uses the rotating electric-field vector of an intense circularly polarized pulse to deflect photo-ionized electrons in the radial spatial direction; the instant of ionization is then mapped to the final angle of the momentum vector in the polarization plane. We resolved subcycle dynamics in tunnelling ionization by the streaking field alone and demonstrate a temporal localization accuracy of 24 as r.m.s. and an estimated resolution of ≈200 as. The demonstrated accuracy should enable the study of one of the fundamental aspects of quantum physics: the process of tunnelling of an electron through an energetically forbidden region.
- P. Eckle, M. P. Smolarski, P. Schlup, J. Biegert, A. Staudte, M. Schöffler, H. G. Muller, R. Dörner, U. Keller, “Attosecond angular streaking”, Nature Phys. 4, 565 (2008)
Laser Electron Diffraction Imaging
Laser-induced electron diffraction is an evolving tabletop method, which aims to image ultrafast structural changes in gas-phase polyatomic molecules with sub-Ångström spatial and femtosecond temporal resolution. Here, we provide the general foundation for the retrieval of multiple bond lengths from a polyatomic molecule by simultaneously measuring the C-C and C-H bond lengths in aligned acetylene. Our approach takes the method beyond the hitherto achieved imaging of simple diatomic molecules and is based upon the combination of a 160 kHz mid-IR few-cycle laser source with full three-dimensional electron-ion coincidence detection. Our technique provides an accessible and robust route towards imaging ultrafast processes in complex gas phase molecules with atto- to femto-second temporal resolution.
- M. Pullen, B. Wolter, A.-T. Le, M. Baudisch, M. Hemmer, A. Senftleben, C. D. Schröter, J. Ullrich, R. Moshammer, C.-D. Lin, J. Biegert, “Imaging aligned polyatomic molecules with laser-induced electron diffraction”, Nature Communications 6, 7262 (2015).
- B. Wolter, M. Pullen, M. Baudisch, M. Sclafani, M. Hemmer, A. Senftleben, C.D. Schröter, J. Ullrich, R. Moshammer, J. Biegert, “Strong-field physics with mid-IR fields”, Phys. Rev. X 5, 021034 (2015).
- B. Wolter, M. G. Pullen, A.-T. Le, M. Baudisch, K. Doblhoff-Dier, A. Senftleben, M. Hemmer, C. Dieter Schröter, J. Ullrich, T. Pfeifer, R. Moshammer, S. Gräfe, O. Vendrell, C. D. Lin, J Biegert, “Ultrafast electron diffraction imaging of bond breaking in di-ionized acetylene”, Science 354, 308 (2016).
Tunneling Dynamics
Strong-field ionisation surprises with richness beyond current understanding despite decade long investigations. Ionisation with mid-IR light has promptly revealed unexpected kinetic energy structures that seem related to unanticipated quantum trajectories of the electrons. We measure first 3D momentum distributions in the deep tunneling regime (c 5 0.3) and observe surprising new electron dynamics of near-zero momentum electrons and extremely low momentum structures, below the eV, despite very high quiver energies of 95 eV. Such level of high-precision measurements at only 1 meV above the threshold, despite 5 orders higher ponderomotive energies, has now become possible with a specifically developed ultrafast mid-IR light source in combination with a reaction microscope, thereby permitting a new level of investigations into mid-IR recollision physics.
- J. Dura, N. Camus, A. Thai, A. Britz, M. Hemmer, M. Baudisch, A. Senftleben, C.D. Schröter, J. Ullrich, R. Moshammer, J. Biegert, “Ionization with low-frequency fields in the tunneling regime”, Scientific Rep. 3, 2675 (2013)
- M. G. Pullen, J. Dura, B. Wolter, M. Baudisch, M. Hemmer, N. Camus, A. Senftleben, C. D. Schroeter, R. Moshammer, J. Ullrich, J. Biegert, “Kinematically complete measurements of strong field ionisation with mid-IR pulses”. J. Phys. B. invited paper, 47, 204010 (2014)
- B. Wolter, C. Lemell, M. Baudisch, M. G. Pullen, X.-M. Tong, M. Hemmer, A. Senftleben, C. D. Schröter, J. Ullrich, R. Moshammer, J. Biegert, J. Burgdörfer, “Formation of very low energy states crossing the ionization threshold of argon atoms in strong mid-infrared fields”, Phys. Rev. A. 90, 063424 (2014)
- B. Wolter, M. Pullen, M. Baudisch, M. Sclafani, M. Hemmer, A. Senftleben, C.D. Schröter, J. Ullrich, R. Moshammer, J. Biegert, “Strong-field physics with mid-IR fields”, Phys. Rev. X 5, 021034 (2015)
Molecular Quantum Control
A coherent control scheme for the population distribution in the vibrational states of nonpolar molecules is proposed. Our theoretical analysis and results of numerical simulations for the interaction of the hydrogen molecular ion in its electronic ground state with an infrared laser pulse reveal a selective two-photon transition between the vibrational states via a coupling with the first excited dissociative state. We demonstrate that for a given temporal intensity profile the population transfer between vibrational states, or a superposition of vibrational states, can be made complete for a single chirped pulse or a train of chirped pulses, which accounts for the accumulated phase difference due to the ac Stark effect. Effects of a spatial intensity (or focal) averaging are discussed.
- A. Picon, J. Biegert, A. Jaron-Becker, A. Becker, “Coherent Control of Vibrational State Population in a Non-Polar Molecule”, Phys. Rev. A 83, 023412 (2011)
- J. Dura, A. Grün, P. Bates, S. Teichmann, T. Ergler, A. Senftleben, T. Pflüger, C.D. Schröter, R. Moshammer, J. Ullrich, A. Jaron-Becker, A. Becker, J. Biegert, “On the wavelength dependence of the suppressed ionization of molecules in strong laser fields”, invited paper, J. Phys. Chem. A 116, 2662–2668 (2012)
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