X-RAY AND UV PHOTOEMISSION-STUDIES OF MONOLAYERS, BILAYERS AND MULTILAYERS OF PHYSISORBED MOLECULES - O-2 AND N-2 ON GRAPHITE

X-ray and UV photoelectron spectroscopy (XPS and UPS) are used to study oxygen and nitrogen molecules physisorbed on graphite. The photoemission spectra from the physisorbed molecules resemble the corresponding spectra for the free molecule. However, the XPS (UPS) binding energies for the monolayer are lowered by about 1.4 +/- 0.1 eV (1.2 +/- 0.2 eV) due to external screening from the polarizable surface and neighbouring molecules. The growth mode of mono-, bi- and multilayers of oxygen can be understood from the layer-dependent binding energy shifts of the adsorbate levels in the XP and UP spectra. The results are compared to XP spectra recorded for N2/graphite. It is found that the binding energy shifts between the first and second layer is 0.50 +/- 0.05 eV for XPS and 0.3 +/- 0.1 eV for UPS, indicating that the final state photohole distribution is of importance. The vibrational progression in the UP spectra from the outermost O2 level, 1pi(g)-1, is observed to be slightly modified for the physisorbed molecules. The modification is more pronounced for the layer closest to the surface than for the outermost layers in the case of multilayers.