  A comprehensive understanding of spin-polarized photoemission is crucial for
accessing the electronic structure of spin-orbit coupled materials. Yet, the
impact of the final state in the photoemission process on the photoelectron
spin has been difficult to assess in these systems. We present experiments for
the spin-orbit split states in a Bi-Ag surface alloy showing that the
alteration of the final state with energy may cause a complete reversal of the
photoelectron spin polarization. We explain the effect on the basis of ab
initio one-step photoemission theory and describe how it originates from linear
dichroism in the angular distribution of photoelectrons. Our analysis shows
that the modulated photoelectron spin polarization reflects the intrinsic spin
density of the surface state being sampled differently depending on the final
state, and it indicates linear dichroism as a natural probe of spin-orbit
coupling at surfaces.
