The optical band-gap energy of a nanostructured tungsten trioxide film is determined using the photoacoustic spectroscopy method under continuous light excitation. The mechanism of the photoacoustic signal generation is discussed. The band-gap energy is also computed by other methods. The absorption coefficient as well as the band-gap energy of three different crystal structures of tungsten trioxide is calculated by a first-principles Green’s function approach using the projector augmented wave method. The theoretical study indicates that the cubic crystal structure shows good agreement with the experimental data.

In this study the relations among nonbridging oxygen (NBO), optical properties, optical basicity, and color center formation in CaO–MgO aluminosilicate glasses were studied. Samples containing (in $\text{mol}\%$ ) 35.9–57.5 of CaO, 16–27.7 of ${\text{Al}}_2{\text{O}}_3$ , 7.9–41.6 of ${\text{SiO}}_2$ , and 6.5–6.9 of MgO were measured by optical absorption and excitation, luminescence, and Raman spectroscopy. The results showed that when the ${\text{SiO}}_2$ content was increased, the absorption edge shifted toward lower wavelengths and the bonds between ${\text{O}}^{2-}$ ions and cations became more covalent. These observations were confirmed by Raman results that showed a decrease in the number of NBO per silicon tetrahedron as a function of ${\text{SiO}}_2$ content. The results indicate that the effects of higher NBO concentration are the narrowing of the band gap energy and the delocalization of ${\text{O}}^{2-}$ electrons, which facilitates the ${\text{O}}^{2-}$ electrons to be trapped by anion vacancies and, consequently, forming color centers. The relationship between color center formation and ${\text{SiO}}_2$ content was confirmed by optical spectroscopic measurements under UV radiation.

The aim of this study was to investigate the diffusion dynamics of 25% hydrogen peroxide (H₂O₂) through enamel-dentin layers and to correlate it with dentin’s structural alterations. Micro-Raman Spectroscopy (MRS) and Fourier Transform Infrared Photoacoustic Spectroscopy (FTIR-PAS) were used to measure the spectra of specimens before and during the bleaching procedure. H₂O₂ was applied to the outer surface of human enamel specimens for 60 minutes. MRS measurements were performed on the inner surface of enamel or on the subsurface dentin. In addition, H₂O₂ diffusion dynamics from outer enamel to dentin, passing through the dentin-enamel junction (DEJ) was obtained with Raman transverse scans. FTIR-PAS spectra were collected on the outer dentin. MRS findings revealed that H₂O₂ (O-O stretching µ-Raman band) crossed enamel, had a more marked concentration at DEJ, and accumulated in dentin. FTIR-PAS analysis showed that H₂O₂ modified dentin’s organic compounds, observed by the decrease in amides I, II, and III absorption band intensities. In conclusion, H₂O₂ penetration was demonstrated to be not merely a physical passage through enamel interprismatic spaces into the dentinal tubules. H₂O₂ diffusion dynamics presented a concentration gradient determined by the chemical affinity of the H₂O₂ with each specific dental tissue.