Soft-x-ray response of transition-metal layer compounds

Abstract

High-resolution soft-x-ray absorption spectra (photon energy 30-200 eV) are presented for the layer compounds Mo${\mathrm{S}}_2$, Nb${\mathrm{Se}}_2$, Ta${\mathrm{S}}_2$, and Ti${\mathrm{S}}_2$. The data were obtained by observing the transmission of synchrotron radiation through small cleaved crystals using a grazing-incidence grating spectrometer. Emphasis is placed on the sulfur $L_{\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}}$ and the selenium $M_{\mathrm{V},\mathrm{I}\mathrm{V}}$ threshold spectra. Details very near threshold are compared with the results of recent band calculations. In Nb${\mathrm{Se}}_2$ the various unoccupied "$d$ bands" can be identified and energy scales established. The $1T$ and $2H$ forms of Ta${\mathrm{S}}_2$ show slightly different sulfur $L$ spectra. Distinct features also appear at the corresponding threshold for Ti${\mathrm{S}}_2$, and these can be identified with excited molecular-orbital states. The sulfur $L$ edge (162 eV) rises most steeply in Mo${\mathrm{S}}_2$ probably because of exciton effects.