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

W-C Lin,F Bisio,J Kirschner,C-T Chiang,A Winkelmann

doi:10.1088/1367-2630/12/8/083022

Abstract

We have investigated the influence of controlled modifications of the surface electronic structure of Cu(001) on the spin polarization of photoelectrons emitted via multi-photon excitation. Using ultrashort, circularly polarized laser pulses with ∼3 eV 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 spin-resolved multi-photon photoemission experiments can be exploited to obtain insights into the mechanism of population of excited states.

Highlights

We have investigated the influence of controlled modifications of the surface electronic structure of Cu(001) on the spin polarization of photoelectrons emitted via multi-photon excitation
Three-photon photoemission (3PPE) from Cs/Cu(001) The general mechanism for the multi-photon coupling between d-bands and image potential (IP) state in normal emission [16, 18] is illustrated in figure 3, where we show the relativistic Cu bulk band structure along the [001] surface normal direction [19] together with the positions of the n = 1 IP state [20] and a relatively broad unoccupied surface resonance observed by inverse photoemission [21]
The vertical lines indicate a possible resonant condition for 3.1. Three-photon photoemission (3PPE) for photon energies around 3 eV, coupling the SO-split d-bands with the IP state

Summary

Introduction

We have investigated the influence of controlled modifications of the surface electronic structure of Cu(001) on the spin polarization of photoelectrons emitted via multi-photon excitation. Circularly polarized laser pulses with ∼3 eV 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. The first approach (figure 1(b)) consists in tuning the energy level of the n = 1 IP state to control the resonance conditions with the SOsplit initial states via the deposition of a submonolayer Cs coverage on the Cu(001) surface. The second approach (figure 1(c)) is to exploit the free-electron-like dispersion of the n = 1 IP state as a function of surface-parallel momentum k [16] to achieve resonant coupling with the d-bands off the surface Brillouin zone (BZ) centre. The orientation of the magnetization vector in the spin detector was kept unchanged with respect to the normal emission case (see below)

Results

Discussion

Conclusion