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Although it has been widely used to study the surfaces of materials (see [24]), X‐ray photoelectron spectroscopy (XPS) can also provide information on the bulk structure. Its application to glasses has been relatively limited to date but it can determine quantitatively the NBO and BO concentrations as well as the concentration of free oxygen (O²⁻), the oxygens not bound to a network former. The technique measures the kinetic energy of photoelectrons that are ejected from the sample (a mm‐glass chip with a freshly exposed surface) as a result of ionization of an element by an incident X‐ray beam. The kinetic energy of the photoelectron can be simply related to the binding energy (BE) of the electron to the nucleus so that it is characteristic of both the element from which it has been ejected and the environment around the element. One can scan for the presence of specific elements and obtain high‐resolution spectra of well‐resolved energy ranges. The concentration of the different oxygen species are determined by curve fitting of the O 1s XPS spectrum [25]. The BO BE is at higher energy (Δ ~ 1.4 eV) than the NBO peak but their FWHM are comparable (average ~ 1.22 eV). The free oxygen peak is incorporated into that of the NBO to the low‐energy side and is observed as an increase in the FWHM of the NBO peak. This is shown in Figure 12 for a PbO–SiO₂ glass. The NBO peak has two contributions, one from the NBOs and one from the presence of free oxygen (O²⁻), also called non‐network oxygen.

Figure 11 Contributions of Fe²⁺ (light gray) and Fe³⁺ (dark gray) to the Mössbauer spectrum of a Fe‐containing borosilicate glass as determined from fits made with 2‐d Gaussian distributions [15].