[show abstract][hide abstract] ABSTRACT: Strong 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.
[show abstract][hide abstract] ABSTRACT: Recollision experiments have been very successful in resolving attosecond scale dynamics. However, such schemes rely on the single atom response, neglecting the macroscopic properties of the interaction and the effects of using multi-cycle laser fields. In this paper we perform a complete spatio-spectral analysis of the high harmonic generation process and resolve the distribution of the subcycle dynamics of the recolliding electron. Specifically, we focus on the measurement of ionization times. Recently, we have demonstrated that the addition of a weak, crossed polarized second harmonic field allows us to resolve the moment of ionization (Shafir, 2012) . In this paper we extend this measurement and perform a complete spatio-spectral analysis. We apply this analysis to reconstruct the ionization times of both short and long trajectories showing good agreement with the quantum path analysis.
Chemical Physics 03/2013; 414:176-183. · 1.96 Impact Factor
[show abstract][hide abstract] ABSTRACT: We probe the dynamics of tunnel ionization via high harmonic generation.
We characterize the ionization dynamics in helium atoms, and apply our
approach to resolve subtle differences in ionization from different
orbitals of a CO2 molecule.
[show abstract][hide abstract] ABSTRACT: Recollision processes provide direct insight into the structure and dynamics
of electronic wave functions. However, the strength of the process sets its
basic limitations - the interaction couples numerous degrees of freedom. In
this Letter we decouple the basic steps of the process and resolve the role of
the ionic potential which is at the heart of a broad range of strong field
phenomena. Specifically, we measure high harmonic generation from argon atoms.
By manipulating the polarization of the laser field we resolve the vectorial
properties of the interaction. Our study shows that the ionic core plays a
significant role in all steps of the interaction. In particular, Coulomb
focusing induces an angular deflection of the electrons before recombination. A
complete spatiospectral analysis reveals the influence of the potential on the
spatiotemporal properties of the emitted light.
[show abstract][hide abstract] ABSTRACT: 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 pulses, laser-induced electron diffraction and holography. 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 mechanics--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 resolution and tunnelling delay times. 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 ion. 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 dynamics. Our approach provides a general tool for time-resolving multi-electron rearrangements in atoms and molecules--one of the key challenges in ultrafast science.