Spin-resolved two-photon photoemission study of the surface resonance state on Co/Cu(001)

Bulk and surface states of a clean and Cs-doped surface of a Co film grown on Cu(001) have been studied by spin-resolved photoemission (SR-PE) and compared with band structure calculation results. One-photon (1PPE) and two-photon (2PPE) photoemission spectra from clean Co films are found to be dominated by a peak located at a binding energy of about 0.4 eV with respect to E-F, which is assigned to the spin up 3d bulk state. Slight Cs-doping of a Co(001) surface shifts an image potential state in resonance with the sp-states of the conduction band. SR-2PPE study of the optically-induced electron population in such an image resonance reveals a strong dependence on the set polarization of the laser light. We are able to directly detect the spin polarization of electrons photoemitted from the image resonance state, which can be varied from highly-polarized (about bulk values) to almost unpolarized when tuning light polarization of the pump laser pulse from s to p.