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Sökning: WFRF:(Kelkensberg F.)

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1.
  • Sansone, G., et al. (författare)
  • Electron localization following attosecond molecular photoionization
  • 2010
  • Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 465:7299, s. 3-763
  • Tidskriftsartikel (refereegranskat)abstract
    • For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10(-15)-s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale(1-4) has become possible only with the recent development of isolated attosecond (10(-18)-s) laser pulses(5). Such pulses have been used to investigate atomic photoexcitation and photoionization(6,7) and electron dynamics in solids(8), and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H-2 and D-2 was monitored on femtosecond timescales(9) and controlled using few-cycle near-infrared laser pulses(10). Here we report a molecular attosecond pump-probe experiment based on that work: H-2 and D-2 are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends-with attosecond time resolution-on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump-probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born-Oppenheimer approximation.
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2.
  • Kelkensberg, F., et al. (författare)
  • Attosecond Control in Photoionization of Hydrogen Molecules
  • 2011
  • Ingår i: Physical Review Letters. - 1079-7114. ; 107:4
  • Tidskriftsartikel (refereegranskat)abstract
    • We report experiments where hydrogen molecules were dissociatively ionized by an attosecond pulse train in the presence of a near-infrared field. Fragment ion yields from distinguishable ionization channels oscillate with a period that is half the optical cycle of the IR field. For molecules aligned parallel to the laser polarization axis, the oscillations are reproduced in two-electron quantum simulations, and can be explained in terms of an interference between ionization pathways that involve different harmonic orders and a laser-induced coupling between the 1s sigma(g) and 2p sigma(u) states of the molecular ion. This leads to a situation where the ionization probability is sensitive to the instantaneous polarization of the molecule by the IR electric field and demonstrates that we have probed the IR-induced electron dynamics with attosecond pulses.
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3.
  • Kling, M. F., et al. (författare)
  • Attosecond control of electron localization in one- and two-color dissociative ionization of H2 and D2
  • 2008
  • Ingår i: 2008 Conference on Quantum Electronics and Laser Science Conference on Lasers and Electro-Optics, CLEO/QELS. - 9781557528599
  • Konferensbidrag (refereegranskat)abstract
    • We present one-color (IR) and two-color (single attosecond XUV pulse + IR) experiments where the sub-cycle evolution of the electric field of light is used to control the dissociative ionization of hydrogen and deuterium molecules.
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4.
  • Kelkensberg, F., et al. (författare)
  • Molecular Dissociative Ionization and Wave-Packet Dynamics Studied Using Two-Color XUV and IR Pump-Probe Spectroscopy
  • 2009
  • Ingår i: Physical Review Letters. - 1079-7114. ; 103:12
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a combined theoretical and experimental study of ultrafast wave-packet dynamics in the dissociative ionization of H-2 molecules as a result of irradiation with an extreme-ultraviolet (XUV) pulse followed by an infrared (IR) pulse. In experiments where the duration of both the XUV and IR pulses are shorter than the vibrational period of H-2+, dephasing and rephasing of the vibrational wave packet that is formed in H-2+ upon ionization of the neutral molecule by the XUV pulse is observed. In experiments where the duration of the IR pulse exceeds the vibrational period of H-2+ (15 fs), a pronounced dependence of the H+ kinetic energy distribution on XUV-IR delay is observed that can be explained in terms of the adiabatic propagation of the H-2+ wave packet on field-dressed potential energy curves.
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5.
  • Mauritsson, Johan, et al. (författare)
  • Attosecond Electron Spectroscopy Using a Novel Interferometric Pump-Probe Technique
  • 2010
  • Ingår i: Physical Review Letters. - 1079-7114. ; 105:5
  • Tidskriftsartikel (refereegranskat)abstract
    • We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration.
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6.
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7.
  • Sansone, G., et al. (författare)
  • Attosecond excitation of electron wavepackets
  • 2008
  • Ingår i: Quantum Electronics and Laser Science Conference, QELS 2008. - 9781557528599
  • Konferensbidrag (refereegranskat)abstract
    • We present experiments, supported by time-dependent Schrödinger simulations, on the dynamics of Helium bound states after an attosecond excitation in the presence of a strong infrared laser field.
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8.
  • Siu, W., et al. (författare)
  • Attosecond control of dissociative ionization of O(2) molecules
  • 2011
  • Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 84:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate that dissociative ionization of O(2) can be controlled by the relative delay between an attosecond pulse train (APT) and a copropagating infrared (IR) field. Our experiments reveal a dependence of both the branching ratios between a range of electronic states and the fragment angular distributions on the extreme ultraviolet (XUV) to IR time delay. The observations go beyond adiabatic propagation of dissociative wave packets on IR-induced quasistatic potential energy curves and are understood in terms of an IR-induced coupling between electronic states in the molecular ion.
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9.
  • Gademann, G., et al. (författare)
  • Attosecond control of electron-ion recollision in high harmonic generation
  • 2011
  • Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 13
  • Tidskriftsartikel (refereegranskat)abstract
    • We show that high harmonic generation driven by an intense near-infrared (IR) laser can be temporally controlled when an attosecond pulse train (APT) is used to ionize the generation medium, thereby replacing tunnel ionization as the first step in the well-known three-step model. New harmonics are formed when the ionization occurs at a well-defined time within the optical cycle of the IR field. The use of APT-created electron wave packets affords new avenues for the study and application of harmonic generation. In the present experiment, this makes it possible to study harmonic generation at IR intensities where tunnel ionization does not give a measurable signal.
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10.
  • Kelkensberg, F., et al. (författare)
  • XUV ionization of aligned molecules
  • 2011
  • Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 84:5, s. 051404-051404
  • Tidskriftsartikel (refereegranskat)abstract
    • New extreme-ultraviolet (XUV) light sources such as high-order-harmonic generation (HHG) and free-electron lasers (FELs), combined with laser-induced alignment techniques, enable novel methods for making molecular movies based on measuring molecular frame photoelectron angular distributions. Experiments are presented where CO2 molecules were impulsively aligned using a near-infrared laser and ionized using femtosecond XUV pulses obtained by HHG. Measured electron angular distributions reveal contributions from four orbitals and the onset of the influence of the molecular structure.
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