## 3 Results and Discussion
### Luminescence Behavior of Fabricated Sol-Gel Materials
Conversely, an intensity of the \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{1}\) band was rather independent on the symmetry in nearest surrounding of Eu\({}^{3+}\) ions. Hence, the ratio between the \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) and the \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{1}\) emission intensities--well-known as R/O--can play the role as a useful tool for estimating the symmetry in which Eu\({}^{3+}\) ions are located [33; 34]. The R/O-ratio value calculated for prepared xerogels was estimated to be 4.13. Furthermore, as was shown in the inset of Figure 4, the luminescence decay curve registered for precursor xerogels was well-fitted to the mono-exponential function and the estimated luminescence lifetime of the \({}^{5}\)D\({}_{0}\) state was estimated to be \(\tau\) = 0.46 ms. The short luminescence lifetime was strictly related with the structure of fabricated xerogels. The luminescence from the \({}^{5}\)D\({}_{0}\) state was quenched by numerous OH groups in local framework of Eu\({}^{3+}\) ions and such groups originated from silanol Si-OH moieties (3664 cm\({}^{-1}\), 3398 cm\({}^{-1}\)) as well as residual water and organic solvents (3233 cm\({}^{-1}\)) inside the porous silicate network. Since the \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{6}\) energy gap of Eu\({}^{3+}\) ions was equal to \(\Delta\)E = 12,500 cm\({}^{-1}\)[35], only about four OH phonons were required to promote a non-radiative deactivation of the \({}^{5}\)D\({}_{0}\) excited level. Since CF\({}_{3}\)COO\({}^{-}\) anions were in the coordination sphere around Eu\({}^{3+}\) cations, it should be noted that such non-radiative relaxation from the \({}^{5}\)D\({}_{0}\) state could be also caused by C = O groups (1664 cm\({}^{-1}\), eight phonons) and the C-F bond (1193 cm\({}^{-1}\), ten phonons). However, among identified functional groups, we expected that OH moieties could play a major role in non-radiative deactivation. In general, the non-radiative rates of intra-configurational 4f\({}^{\text{m}}\)-4f\({}^{\text{m}}\) transitions of rare earths were exponentially dependent on the energy gap (\(\Delta\)E) and the phonon energy in their nearest surrounding (\(\hbar\omega\)). Such correlation is well-known as the energy gap law and it states that an increase in the non-radiative decay rate is assisted by a decreasing number of phonons needed to cover the energy gap, \(\Delta\)E. Hence, if there is a functional group with high vibrational energy in a local framework of Eu\({}^{3+}\) ions, the probability of non-radiative multiphonon relaxation increases. The phonons with maximum energy in a host are also usually called as effective phonons  
Figure 4: Photoluminescence spectrum (PL) spectra recorded for silicate xerogels. Inset presents the luminescence decay curve of the \({}^{5}\)D\({}_{0}\) state of Eu\({}^{3+}\) (\(\lambda_{\text{exc}}\) = 395 nm, \(\lambda_{\text{em}}\) = 591 nm). (\(\hbar\omega_{\rm max}\)) [36; 37]. The influence of different types of functional groups in Eu\({}^{3+}\)-doped silicate glasses on multiphonon relaxation was discussed in work by [PERSON] et al. [38]. The rates of multiphonon relaxation, kNR, via OH modes (3750 cm\({}^{-1}\), kNR = 1.4\(\times\) 10\({}^{-2}\) s\({}^{-1}\)) and Si-O vibrations (1250 cm\({}^{-1}\), kNR = 9.0\(\times\) 10\({}^{-13}\) s\({}^{-1}\)) clearly indicates that OH groups with maximum phonon energy in studied glasses play a major role in quenching the luminescence from the \({}^{5}\)D\({}_{0}\) state. Moreover, the luminescence lifetime is not dependent only on phonon energy in the nearest framework around the optically active ion, but also on another factors, like symmetry. In general, the lower the symmetry is in the nearest vicinity of Eu\({}^{3+}\) ion, the more allowed are the forbidden f-f transitions. Therefore, taking the symmetry aspect into account, the probability of radiative relaxation is relatively high, which results in a short luminescence lifetime.