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Electron spin polarization in strong-field ionization of Xenon atoms

As a fundamental property of the electron, the spin plays a decisive role in the electronic structure of matter from solids to molecules and atoms, e.g. causing magnetism. Hence studying effects regarding the spin is highly interesting. Even 90 years after the famous Stern-Gerlach experiment was carried out in Frankfurt new questions concerning the spin arise consistently. A few pioneering theoretical works recently predicted that electrons, which were created by ionization of noble gas atoms by an intense infrared laser, are spin polarized. Here, we report on the first experimental detection of electron spin polarization by strong-field ionization of xenon atoms (see Figure 1) and support our results with theoretical analysis and an intuitive explanation of the effect (see Figure 2).
We found up to 30% spin polarization changing its sign with electron energy. This work opens the new dimension of spin to strong-field physics.

Figure 1 : spin polarization of electrons ejected by strong field ionization of Xe parallel to the light propagation direction by circularly polarized laser pulses. The spin polarization is defined as the weighted difference between spin-up and spin-down electrons, see Supplementary Material. Consequential positive values correspond to a surplus of electrons with spin parallel to the propagation axis of the laser. Red rectangles show experimental data for 40 fs, 780 nm pulses. Solid blue curve shows results of numerical simulations. Error bars show statistical errors only.

      a)              b)

Figure 2 A: artist view of ionization process of the Xe 5p-state. The 5p j = 3/2 states of Xe are predominantly ionized. In j = 3/2, |mj|= 3/2 states, the electron angular momentum and its spin are parallel. The initial rotational state of the electron results in an offset momentum in the direction of the streaking momentum imparted on the electron by the laser field. Therefore, different spin states correlate to the different, shifted energy distributions, leading to energy dependent spin polarization. Figure 2 B: theoretical energy distribution of s- and p-states. The intuitive picture in panel A is confirmed by results obtained from the numerical solution of the time-dependent Schrödinger equation, for single active electron in the initial p-state. The effective potential for the electron motion is chosen to fit the 5p j=3/2 ionization potential of Xe. For the initial s-state, this potential was modified to maintain the same binding energy as for the p-state. The heights of the three distributions were normalized to unity.

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Nature Photonics 10, 526-528 (2016) 

Electron spin polarization in strong-field ionization of Xenon atoms
Alexander Hartung, Felipe Morales, Maksim Kunitski, Kevin Henrichs, Alina Laucke, Martin Richter, Till Jahnke, Anton Kalinin, Markus Schöffler, Lothar Ph. H. Schmidt, Misha Ivanov, Olga Smirnova and Reinhard Dörner