Electron recapture process during laser-induced double ionization of atoms and molecules

The knowledge of electron structures and correlated dynamics is fundamentally important for many modern technologies and sciences. With intense femtosecond lasers, strong-field physics provides irreplaceable approaches to probe electronic dynamics of atoms and molecules on their natural timescale. Recent researches in strong field physics revealed that released electrons from atoms and molecules with near-zero kinetic energies may be recaptured by their parent ions, even when they are far away from the parent ions. Such electron recapture is a general process and plays an important role in strong-field interaction of atoms and molecules. Electron recapture during strong field double ionization, referred as frustrated double ionization (FDI), provide a novel way to study multi-electron dynamics. Multi-electron dynamics, including electron- electron interaction and correlation, play essential roles in light interactions with matters, from atoms, molecules to solids. Because of the near-zero kinetic energy of the recapture electron, electronic correlation is expected to have a more significant effect on the FDI process than the usual strong field double ionization. Moreover, electron recapture during the dissociative double ionization of molecules, referred as dissociative FDI, can directly affect the molecular dissociation by populating the recapture electron into certain excited states. However, nowadays researches about electron recapture mainly focus on the single-ionization of atoms, and the studies on FDI are limited to simple molecules, such as H2, D2 and Argon dimer. The multi-electron dynamics of FDI in atoms and the dissociative FDI of more complex molecules are so far rarely investigated because of technical difficulties of distinguishing electron recapture signals from strong field ionization and dissociation signals. In this proposal, using novel detection methods with a reaction microscope that recently demonstrated at Photonics Institute, TU Wien, complete kinematical measurements of all involved particles in FDI can be achieved and therefore allow the disentanglement of the electron recapture signals from other strong field signals. The applicant together with the group led by Prof. Andrius Baltuska propose to quantitatively investigate electron recapture during double ionization of atoms and molecules using temporal shaped laser fields, such as CEP-controlled few-cycle and phase-controlled two-color pulses, to study the electronic correlation in FDI of atoms and molecules, and to achieve the control of molecular dissociative dynamics in dissociative FDI of polyatomic molecules. Additionally, accompanying simulations will be performed to help the understanding and analysis of experimental results.

This project focuses on the electron dynamics in single molecules induced by strong laser pulses. Samples of single molecules are prepared in the gas phase in an ultra-high vacuum chamber. The strong laser pulse refers to a laser pulse with a pulse duration around tens of femtoseconds, with the pulse energy around tens of microjoules. In such a laser field, the force felt by an electron is very high - up to 1~10% of the Coulomb force in the hydrogen atom. Many interesting and fundamental phenomena such as electron tunneling ionization, molecular dissociation, and electron re-scattering happen in this regime. Two of our significant results in this project are (i) a new method was found to quantitatively retrieve the angular dependence of laser-induced electron ionization, (ii) by using this method we retrieved the angular dependence of electron re-scattering. Results (i) and (ii) are explained in the following two paragraphs in more detail. (i) We demonstrated a new time-domain method to quantitatively retrieve the angular dependence of ionization from molecules induced by a strong laser field. This method is based on the ion yields of single ionization and double ionization as a function of time delay in the range of the laser-induced rotational half-revival (20~22 picosecond for CO2: carbon dioxide). The previous methods are based on the angle-scanning to achieve the angular dependence of molecular ionization by measuring the ion yields as a function of the angle between the laser field polarization and the molecular axis, thus they strongly rely on the quality of molecular alignment. Our time-domain method is more robust since it rules out the influence of the alignment quality and therefore provides more quantitatively accurate results than previous approaches. (ii) By using the time-domain method, we retrieved the angular dependence of the single ionization and double ionization of CO2 in the linearly polarized strong laser field. From these results, the angular dependence of electron rescattering is obtained. The angular distribution of electron rescattering shows a peak at 51, which is consistent with early results, but with a much narrower distribution since our method rules out the contribution from the imperfection of molecular alignment and is thus more accurate. We also get the maximum value of the angular dependence (55) for electron rescattering in the case of dissociative double ionization. The different maximal angle between the non-dissociative and dissociative double ionization of CO2 implies that the angular dependence of electron rescattering is affected by the state of the parent ion populated during the first ionization step.