Multilayer Growth of Potassium on a Pt(111) Surface

Potassium has been evaporated from fractions of a monolayer up to more than 60 Å thick films on a Pt(111) single-crystal surface held at 100 K. The K 2p binding energies decrease by 0.7 eV with increasing potassium coverage, with the final values at 293.3 eV(K 2p 3 2) and 296.0 eV (K 2p 1 2) for a monolayer. The onset of multilayer growth is characterized by the appearance of a second K 2p doublet at 294.3 and 297.0 eV. Further increase of the potassium film thickness results in plasmon losses with an excitation energy of 3.85 eV. The Pt 4f spectra of the underlying substrate show the same plasmon structure demonstrating the extrinsic part of the plasmon excitation. The inelastic mean free path (IMFP) for photoelectrons in potassium has been determined by comparing the X-ray photoemission intensity of core levels ofa bulk-like potassium film with that from the clean Pt substrate. The result is an IMFP of 22 Å for K 2p electrons at a kinetic energy of about 950 eV. A quantitative evaluation of the XPS signal intensities allows an absolute determination of the potassium film thickness. The monolayer coverage has been determined to be 5.7 × 1014 cm-2 in fair agreement with the structure model deduced from the corresponding ( 3 × 3)R30° LEED pattern. In addition, ordered LEED patterns have been observed during the build up of the second layer.