The electron spin in strong field tunnelionization
Final Report Abstract
The scientific goal of this project was to experimentally determine the spin polarization of electrons from strong field ionization. This was motivated by theoretical predictions most recently from the group of Olga Smirnova that the ionization of rare gas atoms by an ultrashort circularly polarized laser pulse would lead to spin polarized photoelectrons. To experimentally test this prediction a novel TOF-Mott spectrometer was developed and tested. We combined a tailored time-of-flight imaging spectrometer including an extraction field and lenses with a commercial Mott detector. This system, inspired by field geometries of the COLTRIMS spectrometers designed by the group of the applicant, guides electrons into the commercial Mott detector, which was purchased at SPECs and commissioned. The novel spectrometer system, including µ-Metal shielding and vacuum systems was successfully built in the course of the project. After the successful end of the present project, the TOF-Mott system is now used in a follow up project on spin polarization from chiral molecules which is a subproject of CRC 1319 (“ELCH”). Thus, the investment in hardware will continue to produce scientific results also in the future. With this worldwide unique system, we measured the energy dependence of spin polarization of electrons emitted from Xe atoms. Our results are the first ever observation of spin polarization in strong field ionization. With a series of experiments, we could clarify the mechanism responsible for the spin polarization confirming the theoretical predictions. This underlying mechanism is that strong field ionization probability by circularly polarized light depends on the m quantum number of the orbital. From the two energetically degenerate orbitals of m=+1 and m=-1 the one in which the electrons ring current is opposite to the rotation direction of the ionization field is more likely to be ionized. This can be understood in the tunneling picture in which the electron tunnels through a rotating barrier and the tunneling rate depends on the relative direction between the barrier rotation and the angular phase of the initial state. A second essential ingredient to the process is, that the electron from a specific m state take part of the orbital motion as offset momentum to the continuum. Therefore, electron from m=+1 and m=-1 (identified by their respective spin) show different energies. In our first publication, we reported a spin polarization of up to 30% from ionization of Xe by 800nm laser pulses in agreement with calculations solving the time dependent Schrödinger equation. In a second publication, we used light of 400nm and improved the energy resolution of our spectrometer. This allowed us to resolve the j=1/2 and j=3/2 spin-orbit states. We found that the spin polarization flips sign between these two states directly confirming the different continuum energies for electrons from these energetically degenerate orbitals.
Publications
- Measurement of the spin polarization of electronspin a strong laser field. Journal of Physics: Conference Series 635 (2015) 092069
Alexander Hartung, Alina Laucke, Maksim Kunitski and Reinhard Dörner
(See online at https://doi.org/10.1088/1742-6596/635/9/092069) - Electron spin polarization in strong-field ionization of Xenon atoms. Nature Photonics, 10 (2016) 526
A. Hartung, F. Morales, M. Kunitski, K. Henrichs, A. Laucke, M. Richter, T. Jahnke, A.Kalinin, M. Schöffler, L. Ph. H. Schmidt, M. Ivanov, O. Smirnov, R. Dörner
(See online at https://doi.org/10.1038/nphoton.2016.109) - Electron spin polarization in strong-field ionization and the effect of spin in attoclock measurements. The European Conf. on Lasers and Electro-Optics, Munich (2017) ISBN: 978-1-5090-6736-7
Morales, F.; Kaushal, J.; Hartung, A.; Kunitski, M.; Henrichs, K.; Laucke, A.; Richter, M.; Jahnke, T.; Kalinin, A.; Schoffler, M.; Schmidt, L.; Dorner, R.; Ivanov, M.; Smirnova, O.
(See online at https://doi.org/10.1109/CLEOE-EQEC.2017.8086812) - Spin-and Angular Momentum in Strong-Field Ionization. Phys. Rev. Lett., 120 (2018) 043202
D. Trabert, A. Hartung, S. Eckart, F. Trinter, A. Kalinin, M. Schöffler, L. Ph. H. Schmidt, T. Jahnke, M. Kunitski and R. Dörner
(See online at https://doi.org/10.1103/PhysRevLett.120.043202)
