9 research outputs found
Studying the universality of field induced tunnel ionization times via high-order harmonic spectroscopy
High-harmonics generation spectroscopy is a promising tool for resolving
electron dynamics and structure in atomic and molecular systems. This scheme,
commonly described by the strong field approximation, requires a deep insight
into the basic mechanism that leads to the harmonics generation. Recently, we
have demonstrated the ability to resolve the first stage of the process --
field induced tunnel ionization -- by adding a weak perturbation to the strong
fundamental field. Here we generalize this approach and show that the
assumptions behind the strong field approximation are valid over a wide range
of tunnel ionization conditions. Performing a systematic study -- modifying the
fundamental wavelength, intensity and atomic system -- we observed a good
agreement with quantum path analysis over a range of Keldysh parameters. The
generality of this scheme opens new perspectives in high harmonics
spectroscopy, holding the potential of probing large, complex molecular
systems.Comment: 11 pages, 5 figure
High-order harmonic transient grating spectroscopy of SF6 molecular vibrations
special issue : Ultrafast electron and molecular dynamicsInternational audienceStrong field transient grating spectroscopy has shown to be a very versatile tool in time-resolved molecular spectroscopy. Here we use this technique to investigate the high-order harmonic generation from SF6 molecules vibrationally excited by impulsive stimulated Raman scattering. Transient grating spectroscopy enables us to reveal clear modulations of the harmonic emission. This heterodyne detection shows that the harmonic emission generated between 14 to 26 eV is mainly sensitive to two among the three active Raman modes in SF6, i.e. the strongest and fully symmetric nu 1-A1g mode (774 cm-1, 43 fs) and the slowest mode nu5-T2g (524 cm-1, 63 fs). A time-frequency analysis of the harmonic emission reveals additional dynamics: the strength and central frequency of the nu 1 mode oscillate with a frequency of 52 cm-1 (640 fs). This could be a signature of the vibration of dimers in the generating medium. Harmonic 11 shows a remarkable behavior, oscillating in opposite phase, both on the fast (774 cm-1) and slow (52 cm-1) timescales, which indicates a strong modulation of the recombination matrix element as a function of the nuclear geometry. These results demonstrate that the high sensitivity of high-order harmonic generation to molecularvibrations, associated to the high sensitivity of transient grating spectroscopy, make their combination a unique tool to probe vibrational dynamics
Recommended from our members
Multidimensional high harmonic spectroscopy of polyatomic molecules: detecting sub-cycle laser-driven hole dynamics upon ionization in strong mid-IR laser fields
High harmonic generation (HHG) spectroscopy has opened up a new frontier in ultrafast science, where electronic dynamics can be measured on an attosecond time scale. The strong laser field that triggers the high harmonic response also opens multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. Using combined experimental and theoretical approaches, we show how multi-dimensional HHG spectroscopy can be used to detect and follow electronic dynamics of core rearrangement on sub-laser cycle time scales. We detect the signatures of laser-driven hole dynamics upon ionization and reconstruct the relative phases and amplitudes for relevant ionization channels in a CO2 molecule on a sub-cycle time scale. Reconstruction of channel-resolved complex ionization amplitudes on attosecond time scales has been a long-standing goal of high harmonic spectroscopy. Our study brings us one step closer to fulfilling this initial promise and developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems
Resolving the time when an electron exits a tunnelling barrier
The tunnelling of a particle through a barrier is one of the most fundamental and ubiquitous quantum processes. When induced by an intense laser field, electron tunnelling from atoms and molecules initiates a broad range of phenomena such as the generation of attosecond pulses1, laser-induced electron diffraction2, 3 and holography2, 4. These processes evolve on the attosecond timescale (1 attosecond ≡ 1 as = 10−18 seconds) and are well suited to the investigation of a general issue much debated since the early days of quantum mechanics5, 6, 7--the link between the tunnelling of an electron through a barrier and its dynamics outside the barrier. Previous experiments have measured tunnelling rates with attosecond time resolution8 and tunnelling delay times9. Here we study laser-induced tunnelling by using a weak probe field to steer the tunnelled electron in the lateral direction and then monitor the effect on the attosecond light bursts emitted when the liberated electron re-encounters the parent ion10. We show that this approach allows us to measure the time at which the electron exits from the tunnelling barrier. We demonstrate the high sensitivity of the measurement by detecting subtle delays in ionization times from two orbitals of a carbon dioxide molecule. Measurement of the tunnelling process is essential for all attosecond experiments where strong-field ionization initiates ultrafast dynamics10. Our approach provides a general tool for time-resolving multi-electron rearrangements in atoms and molecules11, 12, 13--one of the key challenges in ultrafast scienc
HHG probing of atomic dipoles by electronic wave-packet caustics
We exploit high-order harmonic generation spectroscopy at the caustics of the recombining electron wave-packet as a method for directly comparing experimental spectra with ab-initio theories. Experimental results in xenon and comparison with ab-initio time-dependent configuration-interaction singles calculations allowed to assess the role of the wave-packet enhancement during the giant resonance. Results in argon show that this technique can also be applied to other targets
HHG probing of atomic dipoles by electronic wave-packet caustics
We exploit high-order harmonic generation spectroscopy at the caustics of the recombining electron wave-packet as a method for directly comparing experimental spectra with ab-initio theories. Experimental results in xenon and comparison with ab-initio time-dependent configuration-interaction singles calculations allowed to assess the role of the wave-packet enhancement during the giant resonance. Results in argon show that this technique can also be applied to other targets
CLEO<sup>®</sup>/Europe — IQEC 2013 looking inside the recollision process
Probing the ionization times and the recollision times by adding a weak perturbation in simple atomic systems was discussed. The results deviated from the simple classical model were found to be in agreement with the quantum path analysis. The multiple channel ionization and the differences between two ionization channels were probed in aligned CO2 molecules.Peer reviewed: YesNRC publication: Ye