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 $6\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ has been measured. At this photon energy of 3.14 eV the above-threshold ionization peaks connected to ${\mathrm{Xe}}^{+}$ ions in the ground state ($J=3/2$, ionization potential $I_p=12.1\mathrm{eV}$) and the first excited state ($J=1/2$, $I_p=13.4\mathrm{eV}$) 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.