Control of electron recollision and molecular nonsequential double ionization

ARTICLE https://doi.org/10.1038/s42005-020-0297-3 OPEN Control of electron recollision and molecular nonsequential double ionization 1, Itzik Ben-Itzhak2 & Marcos Dantus 1,3 ✉ 1234567890():,; Shuai Li1, Diego Sierra-Costa1, Matthew J. Michie Intense laser pulses lasting a few optical cycles, are able to ionize molecules via different mechanisms. One such mechanism involves a process whereby within one optical period an electron tunnels away from the molecule, and is then accelerated and driven back as the laser field reverses its direction, colliding with the parent molecule and causing correlated nonsequential double ionization (NSDI). Here we report control over NSDI via spectral-phase pulse shaping of femtosecond laser pulses. The measurements are carried out on ethane molecules using shaped pulses. We find that the shaped pulses can enhance or suppress the yield of dications resulting from electron recollision by factors of 3 to 6. This type of shaped pulses is likely to impact all phenomena stemming from electron recollision processes induced by strong laser fields such as above threshold ionization, high harmonic generation, attosecond pulse generation, and laser-induced electron diffraction. 1 Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA. 2 J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS 66506, USA. 3 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA. ✉email: COMMUNICATIONS PHYSICS | (2020)3:35 | https://doi.org/10.1038/s42005-020-0297-3 | www.nature.com/commsphys 1 ARTICLE A COMMUNICATIONS PHYSICS | https://doi.org/10.1038/s42005-020-0297-3 tomic ionization processes in the presence of strong laser fields are well modeled by single active electron ionization approximations at moderate intensities1,2. In contrast, non-sequential double ionization (NSDI) is complicated by strong electron correlation and Coulombic attraction to the ion core3–5. Processes analogous to those occurring in atoms take place in molecules, albeit with added complexity introduced by the presence of other atoms, molecular structure, additional degrees of freedom, and higher density of electronic states. Furthermore, unlike atoms, molecules, especially nonlinear polyatomic molecules, can experience electron recollision processes at any of their atoms, making possible the observation of NSDI with elliptical and even circularly polarized fields6–14. Understanding and controlling the behavior of molecules in strong fields requires the ability to discriminate between different double ionization mechanisms. The choice of a molecule with high ionization potential and long-wavelength short-pulse excitation has been shown to favor recollision-induced molecular NSDI3. Shorter wavelengths and longer pulse durations may cause NSDI in addition to sequential multiphoton ionization (MPI), wherein large fragment ions absorb additional photons from the field and undergo further fragmentation and ionization15,16. Controlling the ionization process would be useful to simplify the analysis of molecular ionization in strong fields. Moreover, controlling electron recollision could impact a wide range of phenomena that depend on this process, for example: molecular fragmentation, high-harmonic generation (HHG), the generation of attosecond pulses via HHG, above threshold ionization (ATI), and even attosecond clocking5,17. Control of these processes via pulse shaping has been of interest for over a decade18. Most relevant to the findings here are calculations that have shown that large effects can be gained by ‘jumps’ in the optical cycles of a pulse. For example, a pulse resulting from joining two identical pulses with one having its carrier envelope phase shifted by π, was predicted theoretically to extend the HHG cutoff19. Similarly, a theoretical exploration of pulses with an instantaneous π phase jump in the time domain predicted a significant extension of the HHG cutoff energy and the energies achieve by ATI20,21. Unfortunately, these schemes have not been experimentally implemented. The latter case, for example, would require pulses spanning more than five octaves, a capability that is well beyond the present state of the art in ultrafast science. The purpose of the present study is to enhance or suppress the yield of metastable dications via spectral-phase pulse shaping. Inspired by the elegant control experiments by Silberberg using phase steps to control two-photon excitation22, we use a phase step. However, in this work, laser-matter interactions are well outside the perturbation limit considered by Silberberg. In fact, perturbation theory only predicts a significant reduction in the yield of high-order (five photons or more) processes. The phase step in the frequency domain causes a jump in the time-dependent frequency of the pulse during the time when tunnel ionization and recollision occurs. The jump in frequency is relevant because recollision, NSDI, and HHG have been found to depend on the frequency of the laser elevated to a power of approximately five or eight23–25, stemming from quantum path interferences with contribution from multiple returning orbits. In addition to calculations of the kinetic energy acquired by a free electron in the shaped laser field, we develop a model exclusively based on the fact that the frequency of the shaped pulses varies through the temporal pulse. Thus, we assume the position of the phase step with respect to the center frequency of the pulse and the sign of the phase step in the frequency domain is proportional to the frequency of the pulse at the times when recollision takes place. Here, using laser pulses with identical spectrum and peak intensity, but different phase characteristics, we observe control 2 over the yield of ions produced via NSDI and MPI mechanisms. We find that the model predicts quite closely the observed enhancement or suppression of dications as well as other electron recollision processes such as high-harmonic generation. We test the control mechanism using circularly polarized pulses, which involve longer orbits for the recolliding electron wavepacket and lower recollision energies. We find that the contrast observed for doubly charged fragment ions is slightly larger than that measured for linearly polarized pulses. This indicates that the control mechanism is associated either with the recolliding electron wavepacket orbit period, which is here manipulated by the position and sign of the phase step, or the kinetic energy of the recolliding electron, which is above or below the double ionization threshold due to pulse shaping. In summary, we report the observation of significant control over the yield of metastable dications as we scan a ¾π step across the spectrum of an intense femtosecond laser pulse. The enhancement or suppression of electron wavepacket recollision appears to be universal based on similar phase-step me (...truncated)