Core-level reflectance spectroscopy of black phosphorus single crystals

The $2p$ core reflectance spectra have shown strong anisotropy for $\overrightarrow{\mathrm{E}}\parallel a$ and $\overrightarrow{\mathrm{E}}\parallel c$. A remarkably sharp doublet structure observed at the $2p$ core threshold for $\overrightarrow{\mathrm{E}}\parallel a$ (at 130.33 and 131.20 eV) is assigned to the spin-orbit partner of the $2p$ core excitons with finite binding energy. The difference between the spectra for $\overrightarrow{\mathrm{E}}\parallel a$ and $\overrightarrow{\mathrm{E}}\parallel c$ is explained on the basis of selection rules for transitions from the $2p$ core levels to particular symmetry points ($Z_4^{-}$, ${\Sigma }_1$, and ${\Gamma }_4^{-}$) of conduction bands.