Direct determination of the electronic structure ofS4N4by x-ray and ultraviolet photoemission

The results of a study of the electronic structure of ${\mathrm{S}}_4$ ${\mathrm{N}}_4$ molecules are reported. The photoemission techniques employed include: x-ray photoemission spectroscopy (XPS) of the full valence band of thin films composed of ${\mathrm{S}}_4$ ${\mathrm{N}}_4$ molecules; XPS core-level spectra of ${\mathrm{S}}_4$ ${\mathrm{N}}_4$ molecules in the solid state as well as in the gas phase; and ultraviolet photoemission spectroscopy of the uppermost ionization potentials of ${\mathrm{S}}_4$ ${\mathrm{N}}_4$ molecules in the gas phase. The data are analyzed using a self-consistent-field molecular-orbital calculation in the complete-neglect-of-differential-overlap approximation. Chemical shifts are observed in the ${\mathrm{S}}_4$ ${\mathrm{N}}_4$ gas phase and solid-sample core-level spectra and are used to obtain an estimate of the amount of charge transfer $q\approx \frac{e}{2}$. A polarization energy 1.2 eV is deduced. The optical-absorption spectrum of ${\mathrm{S}}_4$ ${\mathrm{N}}_4$, analyzed using a configuration-interaction treatment of our molecular orbitals, is found to be dominated by molecular excitons.