Hard X-ray photoelectron spectra (HXPES) of bulk non-conductor vitreous SiO2: Minimum linewidths and surface chemical shifts

The Cr coating produces surface monolayers of interfacial Cr “suboxide” (Cr-subox), Cr metal, and a surface Cr oxide (Cr-surfox). Cr-subox (SiOCr) gives rise to the weak near-surface Si 1s peak, while both oxides give rise to both the weak surface O 1s peak and the Cr 2p oxide peak. Both the O 1s and Si 1s surface peaks are shifted by ∼2 eV relative to the large bulk Si 1s and O 1s peaks. The weak Si 1s and O 1s surface peaks along with the Cr 2p oxide peak decrease in intensity greatly as the photon energy increases, due to an increase in inelastic mean free path (IMFP) in SiO2 from 27 Å at 1486 eV to 90 Å at 5000 eV. The decrease in intensity of these surface peaks is consistent with the theoretical IMFP's in the literature. The intensity of the weak O 1s and Si 1s peaks, along with the Cr 2p oxide peak, shows that the surface structure gives close to monolayer amounts of Cr-subox, Cr metal, and Cr-surfox. These high-energy spectra should be very useful in the future for bulk analysis of the O species in non-conducting silicate glasses, or surface analysis on non-conducting silicate surfaces and bulk as a function of photon energy.