Spin and Angular Momentum in Strong-Field Ionization

The spin polarization of electrons from multiphoton ionization of Xe by 395 nm circularly polarized laser pulses at 6×10¹³ W/cm² has been measured. At this photon energy of 3.14 eV the above-threshold ionization peaks connected to Xe⁺ ions in the ground state ($J=3/2$, ionization potential Iₚ=12.1 eV) and the first excited state ($J=1/2$, Iₚ=13.4 eV) are clearly separated in the electron energy distribution. These two combs of above-threshold ionization peaks show opposite spin polarizations. The magnitude of the spin polarization is a factor of 2 higher for the $J=1/2$ than for the $J=3/2$ final ionic state. In turn, the data show that the ionization probability is strongly dependent on the sign of the magnetic quantum number.