Dominance of the first excitation step for magnetic circular dichroism in near-threshold two-photon photoemission

Magnetic circular dichroism (MCD) in near-threshold photoemission is measured for a perpendicularly magnetized Cs/Co/Pt(111) film with work function adjusted by Cs adsorption. For one-photon photoemission (1PPE) the MCD asymmetry is recorded at a fixed photon energy of h nu = 3.06 eV and varying work function Phi. The asymmetry shows a nonmonotonous behavior in dependence of the excess energy h nu-Phi with a maximum value of A(1PPE) = 6.2 % at Phi = 2.45 eV. The measurement explores the first excitation step of a former two-photon photoemission (2PPE) measurement with A(2PPE) = 8.4% demonstrating that in 2PPE from Co(111) the first excitation step is the dominant asymmetry-generating process. An energy-dependent measurement in 2PPE at reduced work function (Phi approximate to 3 eV) yields a constant asymmetry of about 17% in the photon energy range between h nu = 1.53-1.66 eV. It reveals that for Co(111) the involvement of a real intermediate state is crucial for enlarged MCD asymmetries. Both results are discussed in the framework of direct interband transitions in directions deviating from the direction of normal electron emission Gamma-L. The 1PPE measurement is in reasonable agreement with calculations on the basis of this model. This reveals that an ab initio calculation considering all directions of excitation with an additional restriction in energy due to the existence of the sample work function in the photoemission process adequately describes MCD asymmetries in near-threshold photoemission.