Electronic structure of monoclinic selenium (Se8): Comparison withS8and trigonal selenium

High-resolution valence electron energy distributions are reported for monoclinic selenium ($m-\mathrm{S}\mathrm{e}$, i.e.,${\mathrm{Se}}_8$), trigonal selenium, and orthorhombic sulfur ($\sigma -\mathrm{S}$, i.e.,${\mathrm{S}}_8$). These data, as well as previously published optical spectra of ${\mathrm{Se}}_8$ are analyzed using our spectroscopically parametrized CNDO-S (complete neglect of differential overlap, spectroscopically parametrized) molecular-orbital model. The general features of the valence-electron spectra are regarded as direct consequences of the symmetry of the molecular units and the difference in $s$- and $p$-electron ionization potentials. Important details in the $s$-electron photoemission spectra and in the optical-absorption spectra, however, require the full power of the CNDO-S model for their interpretation. We conclude that the photoemission and optical-absorption spectra of $m-\mathrm{S}\mathrm{e}$, like those of $\sigma -\mathrm{S}$, can be interpreted directly in terms of the electronic structure of the individual ${\mathrm{Se}}_8$ and ${\mathrm{S}}_8$ molecules, respectively.