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Femto- and attosecond imaging of molecular multiple ionization: Time-resolved electron and nuclear dynamics using free electron lasers and ultra-short pulses

Final Report Summary - ATTOTREND (Femto- and attosecond imaging of molecular multiple ionization: Time-resolved electron and nuclear dynamics using free electron lasers and ultra-short pulses)

The main objective of the ATTOTREND project is to develop a set of accurate theoretical tools to provide time-resolved images and strategies for control of electron and nuclear dynamics in molecules subject to intense and ultrashort XUV pulses. The interest of this project is focused on single and multi-photon ionization processes that may lead to a full breakup of the molecular target. The detection of the fragments in coincidence that allow for a complete reconstruction of the process can nowadays be achieved with the most advanced detection techniques that are being implemented in experimental laboratories all over the world. Several European laboratories are currently operating at XUV and soft X-ray photon energies (for instance, Tesla Test Facility FEL-Hamburg in Germany, 4GLS-Daresbury in UK, or ELETTRA-Trieste in Italy). Radiation sources based on High-Harmonic Generation (HHG) techniques and Free-Electron Lasers (FEL) are able to generate burst of light reaching durations in the attosecond regime, therefore giving access to explore the actual time scale for electron motion (of the order of a few hundreds of attoseconds). This electron dynamics is followed by using pump-probe schemes where an ultrashort pulse pumps the target creating a wave packet in its excited or ionized states, and its evolution is then traced or controlled by a second pulse, the probe, interacting with the target at different time delays respect to the pump pulse. The shorter the pulse the shorter the timescale of the dynamics that can be later probed in the absence of field.
In this context of unprecedented time and space resolutions, the availability of solid theoretical support becomes essential to understand the underlying mechanisms, and more importantly, to design new approaches to steer and drive the electron and nuclear dynamics. This is the aim of this project. During this grant, we have focused in three main lines:
a) Exploring coupled electron and nuclear motion in singly and doubly excited states of the hydrogen molecule.
We have proposed new schemes to explore electron dynamics in singly excited hydrogen molecule. The state-of-the-art pump-probe schemes involve two XUV pulses as pump and probe, which gives access to the natural dynamics of the target without introducing distortions in the molecular potential. The most outstanding publications in this topic includes two theoretical papers:
A. González-Castrillo et al., Phys. Rev. Lett. 108, 063009 (2012)
A. Palacios et al., PNAS 111, 3973 (2014).
and one publication in collaboration with Prof. D. Charalambidis in Crete (Greece) with the first experimental attempt of an XUV-pump/XUV-probe protocol in molecules:
P. A. Carpeggiani et al., PRA 89, 023420 (2014).
We have also explored the use of XUV-pump/IR-probe schemes to control the electron dynamics launched in the singly excited molecule. We should highlight our work performed in collaboration with the group of Prof. Murnane and Kapteyn in JILA, University of Colorado (USA) and published in PNAS 111, 912 (2014). We investigated the dynamics of electronic wave packets generated by an attosecond pulse train and a phase-locked IR pulse containing optically allowed and optically forbidden states. By probing with a second IR pulse one can control the ionization outcome.
Finally, we have developed a new model to describe the dynamics of doubly excited states in molecules. These are metastable states, highly correlated states embedded in the ionization continua, which decay into its electronic background after a given time. While in atoms this is a well-described phenomena leading to the well-known Fano lineshapes, in molecules the role of nuclear motion may strongly modify the picture. This model was published in:
“Autoionization of molecular hydrogen: where do the Fano lineshapes go?”, A. Palacios, J. Feist, A. González-Castrillo, J. L. Sanz-Vicario and F. Martín, Chem. Phys. Chem. 14, 1456 (2013).
And later extended for angular distributions and employed in a collaboration with the experimental group of Prof. R. Moshammer in Heidelberg, leading to a common work published in Phys. Rev. Lett. 110, 213002 (2013).
This work followed our investigations (in the first period of the project) on the time-resolved picture of the decay of doubly excited states in H2 and D2, published as:
“Reproducibility of Observables and Coherent Control in Molecular Photoionization: From cv to Ultrashort Pulsed Radiation”, A. González-Castrillo, A. Palacios, F. Catoire, H. Bachau and F. Martín, J. Phys. Chem. A 116, 2704 (2012)
Following these works, two book chapters have been written reviewing
i) UV pump-IR probe techniques applied to h2 and D2:
- “Probing electron dynamics in simple molecules with attosecond pulses”, P. Rivière, A. Palacios, J. F. Pérez-Torres and F. Martín, Progress in Ultrafast Intense Laser Science VIII, Springer Series in Chem. Phys., Vol. 103, Eds.: K. Yamanouchi, M. Nisoli and W. T. Hill (2012)
and
ii) the use of UV pump – UV probe techniques leading to high-intense effects and control of the ratio of dissociative and non-dissociative ionization channels:
- “XUV lasers for ultrafast electronic control in H2”, A. Palacios el al., Ultrafast Phenomena in Molecular Sciences. Femtosecond Physics and Chemistry. 107, 25 (2014).
b) Developing new methodologies to explore one-electron molecular targets subject to intense laser pulses.
We have made significant progresses in the implementation of a new code to explore H2+ subject to strong fields. Part of this material remains to be soon published. Within this context, however, we have established a collaboration with the group of prof. I. Solá in the Universidad Complutense de Madrid, investigating control schemes for the creation of giant dipoles in H2+ subject to intense pulsed radiation. This work has lead to three publications Chem. Phys. Chem. 14, 1405 (2013); J. Chem. Phys. 139, 084306 (2013) and J. Phys. B 48, 043001 (2015).

c) Equivalent time-dependent implementations of the previous methods has been applied in combination with existing DFT-like tools to study photoionization of larger multielectronic targets: small (CO, BF3, CF4) and large molecules (amino acids). The work was developed in collaboration with the theoretical group of Prof. P. Decleva and with several experimental groups.
For small molecules, experiments were performed in SOLEIL (France) and SPRING-8 (Japan). We found that the intramolecular scattering is responsible for the observed oscillations in the ratios between vibrationally resolved photoionization cross sections. Moreover, we demonstrated that these patterns allow one to extract structural information of the molecule. The work has lead to four publications: J. Phys. B 47, 124032 (2014); J. Elec. Spec. And Rel. Phen. 195, 320 (2014); Phys. Rev. A 88, 033412 (2013) and J. Chem. Phys. 139, 124306 (2013).
For the large molecules, specifically the amino acid phenylalanine, experiments requiring single attosecond pulses were carried out by the group of prof. M. Nisoli in Milano (Italy) and the work has been recently published in Science 346, 336 (2014). We found that charge migration takes place in a few fs, before the nuclei have time to move. Work is still in progress in this line.