Spin-dependent lifetime and exchange splitting of surface states on Ni(111)

We report on a spin-resolved two-photon photoemission study of the Ni(111) surface states. Nickel thin films were grown by molecular beam epitaxy on a W(110) substrate. The first image-potential state is used as a sensor to map the spin polarization of the occupied surface states. This allows us to identify the majority spin component of the Shockley surface state as well as a majority and minority d-derived surface resonance. The n = 1 image-potential state is found to be exchange split by 14 +/- 3 meV. In spite of the fact that the band structure at the Fermi level exhibits a strongly discerned density of states in both spin channels, we observe low spin asymmetries in the decay and dephasing rates of the photoexcited electrons. Varying the sample preparation reveals that the Shockley surface state contributes about 40% to the spin-dependent decay rate.