Optical and electron-energy-loss spectroscopy of GeS, GeSe, SnS, and SnSe single crystals

From a combined analysis of optical transmission and electron-energy-loss spectra, we derive the $\ensuremath{\epsilon}$ spectra for the two principal polarizations parallel to the layer plane from 0.5 to 22 eV for GeS and GeSe, and from 0.5 to 28 eV for SnS and SnSe. Calculated reflectivity spectra are in good agreement with the experimental ones from 1.5 to 6 eV. Interband transitions are assigned with help of the recent band-structure calculation of Grandke. The plasmon energies are shifted differently by $\mathrm{Ge} 3d$ and $\mathrm{Sn} 4d$ excitations. The plasmon dispersion is free-electron-like and isotropic.