Spin Polarization in Strong-Field Ionization as Sensor of Trapped Electron Orbits.

We show how spin polarization of photoelectrons produced in strong-field ionization of noble-gas atoms can be used to detect highly unexpected electron orbits created in strong, circularly polarized near-infrared fields. In such fields, ionization is expected to generate photoelectrons rapidly moving away from the ionic core, freezing their initial spin due to the short-range nature of the spin-orbit interaction between the photoelectron and the core. However, we find clear evidence of spin flips in the photoelectron spectra at low photoelectron energies and confirm that they happen because a photoelectron has been trapped near the core. The presence of such highly unexpected orbits explains the marked deviations between the recent experimental measurements of spin polarization [Nat. Photonics 10, 526 (2016)NPAHBY1749-488510.1038/nphoton.2016.109] and simulations with frozen spin.