X-ray photoelectron spectra and electronic structure of zirconium trisulfide and triselenide

X-ray photoelectron spectra of ZrS₃ and ZrSe₃ have been recorded. These compounds may be regarded as Zr(Ch₂)Ch (Ch=S,Se) with both (Ch₂) groups and isolated Ch atoms. An unambiguous assignment of the chalcogen core levels was made by comparing the spectra with those of ZrS₂ and ZrSe₂ (where all chalcogen atoms are isolated). The levels of the (Ch₂) groups are found at higher binding energies than those of the isolated Ch atoms, which is consistent with a larger negative charge on the isolated atoms. The structures of the valence bands of ZrS₃ and ZrSe₃ are discussed in terms of a molecular-orbital scheme for the (Ch₂) groups. The exciton peaks observed in the optical absorption spectra of ZrS₃ and ZrSe₃ are assigned as due to excitations of the (Ch₂) groups.