Modulations of valence-band photoemission spectrum fromC60monolayers on Ag(111)

The photon-energy dependences of valence-band photoemission spectra have been investigated for monolayer, submonolayer, and multilayer ${\mathrm{C}}_{60}$ films on Ag(111). The intensity ratios of the highest and next-highest occupied molecular orbitals (the HOMO and NHOMO) of the films are found to oscillate with the same period as the incident photon energy is varied, but the amplitude of the oscillation is smaller with lower film coverage. Since the characteristics of the molecular orbitals of ${\mathrm{C}}_{60}$ in the monolayers are considerably different from those of the thick film due to the charge-transfer interaction between the metallic surface and the molecule the observation opposes the proposal in which the oscillation is entirely dependent on the nature of the empty and occupied states involved in the photoemission process. The results instead support the photoelectron interference model by Hasegawa et al. [Phys. Rev. B 58, 4927 (1998)]. Variation of the oscillation amplitude with ${\mathrm{C}}_{60}$ coverage is explained in terms of hybridization of the HOMO and NHOMO with the surface. Depositing ${\mathrm{C}}_{60}$ onto Ag(111) held at a high temperature produces a single-phase $(2\mathrm{}\sqrt{3}\times 2\sqrt{3})$ $R30\mathrm{}°$ ${\mathrm{C}}_{60}$ monolayer. Angle-resolved photoemission experiments reveal changes in the relative intensities of the two bands with the azimuth-emission angle, measured with respect to the symmetry axes of the ordered monolayer. The intensity modulation is due to anisotropic scattering of photoelectrons along various molecular axes.