π* and σ* molecular orbitals of condensed films of chlorobenzenes and hexafluorobenzene observed by inverse photoemission

The electron affinity levels of condensed films of benzene, chlorobenzene, 1.4-dichlorobenzene, s-trichlorobenzene, and hexafluorobenzene on Cu(111) have been determined by isochromat inverse photoemission spectroscopy. In addition to the π* orbitals of the benzene ring, low-lying σ* orbitals are observed in the chlorine and fluorine-substituted molecules. Comparison to electron transmission spectra allows the assignment of the unoccupied molecular orbitals and the determination of the relaxation energies in going from the gas phase to the solid state. These relaxation energies are found to decrease in the series of benzene to trichlorobenzene.