Femtosecond-Laser Photoemission Spectroscopy of Mo(100) Covered by Ultrathin MgO(100) Films of Variable Thickness

Ferntosecond-laser photoemission spectra were obtained from a Mo(100) single crystal surface covered with stoichiometric magnesia thin films of well-defined thickness. Depending on the excitation wavelength and the MgO layer thickness, either one or two photon photoemission was detected at 333 and at 402 nm. This photoemission was assigned to originate from the molybdenum d band states even for 10 monolayers MgO coverage. At 263 nm excitation, however, the photoemission power dependence indicated that the signal contained both contributions from the Mo metal d electrons and from the MgO O2p valence band. While one nominal monolayer of MgO only slightly influenced the photoemission from the molybdenum surface, a considerable reduction of the surface electronic work function was observed for ultrathin magnesia overlayers of 2 to 3 monolayers in thickness. No significant change in the work function was measured for more than 3 up to 10 monolayers.