Spin-polarized multi-photon photoemission and surface electronic structure of Cu(001)

We have investigated the influence of controlled modifications of the surface electronic structure of Cu(OOl) on the spin polarization of photoelectrons emitted via multi-photon excitation. Using ultrashort, circularly polarized laser pulses with ~3eV photon energy, spin-polarized electrons can be selectively excited from the spin-orbit (SO) coupled d-bands of Cu into the unoccupied n = 1 image potential (IP) state on the Cu(001) surface. Upon lowering the IP state energy level by submonolayer Cs deposition, we show that the IP energy can be tuned into two-photon resonance with initial state d-bands of different double group symmetry, leading to a sign reversal of the spin polarization that is observed at the IP state level, Similarly, exploiting the parallel-momentum IP state dispersion, the resonant tuning of the IP state energy level to different branches of the SO split d-bands is demonstrated. Our results highlight the role of resonant and off-resonant excitation pathways in determining the spin polarization in the excited states. The additional information contained in spinresolved multi-photon photoemission experiments can be exploited to obtain insights into the mechanism of population of excited states.