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 stud y oxygen and nitrogen molecules physisorbed on graphite. The photoemis sion spectra from the physisorbed molecules resemble the corresponding spectra for the free molecule. However, the XPS (UPS) binding energie s for the monolayer are lowered by about 1.4 +/- 0.1 eV (1.2 +/- 0.2 e V) due to external screening from the polarizable surface and neighbou ring molecules. The growth mode of mono-, bi- and multilayers of oxyge n can be understood from the layer-dependent binding energy shifts of the adsorbate levels in the XP and UP spectra. The results are compare d 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 ph otohole distribution is of importance. The vibrational progression in the UP spectra from the outermost O2 level, 1pi(g)-1, is observed to b e slightly modified for the physisorbed molecules. The modification is more pronounced for the layer closest to the surface than for the out ermost layers in the case of multilayers.