## 3 Results and Discussion
### Photoluminescence Analysis  
RT PLE and selectively excited PL spectra of the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\):Cr spheres are shown in Figure 4. The PL spectra are dominated by two narrow lines (the above-mentioned R-lines) in the red spectral region, which are due to the \({}^{2}\)E \(\rightarrow\)\({}^{4}\)A\({}_{2}\) transition (see the Tanabe-Sugano diagram in Figure 4c and energy level scheme in Figure 4b). These lines appear with a separation of 3.6 meV from each other, the first one placed at 1.792 eV (R\({}_{2}\)) and the other at  
Figure 3: (**a**) XRD diffractogram of the powdered sample demonstrating its monophasic nature; (**b**) Raman spectrum performed with 441.6 nm laser line of a HeCd laser. Figure 2: (**a**) Optical photograph of the ruby sphere when illuminated with a UV lamp (365 nm); (**b**) electron microscopy image of the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) sphere surface; (**c**) high-magnification SEM image evidencing the polycrystalline nature at the outer sphere. 1.788 eV (R\({}_{1}\)). This observation agrees well with the values reported for individual Cr\({}^{3+}\) in Al substitutional sites identified in dilute doped samples [3,7,8]. As above-mentioned, the observation of the two R-lines indicates that, in the studied samples, the trivalent chromium ions are in distorted octahedral sites and their energy levels are split by a low symmetry trigonal component of the crystal field and spin-orbit coupling. At RT, the FWHM of the R-lines is about 2.5 meV. On the lower and higher energies sides of the zero phonon R-lines, a vibrationally assisted structure can be identified, corresponding to the Stokes and anti-Stokes emission, respectively. In addition, on the low energy side of the R-lines, sharp features can be discerned at ca. 1.77 eV and 1.76 eV, assigned to the presence of chromium ion pairs and/or Cr\({}^{3+}\)-complex defects, the so-called, N-lines (N\({}_{2}\) and N\({}_{1}\), respectively, due to fourth-nearest neighbor and second-nearest neighbor pairs, respectively), whose intensity ratio to the R-lines is known to be dependent on the chromium concentration [45, 46, 47, 48, 49]. On the high-energy side of the R-lines, two zero-phonon lines separated by ca. 27 meV and located at 1.882 eV and 1.855 eV, respectively, are well-resolved. These, called R\({}^{\prime}\)-lines, are assigned to the \({}^{2}\)T\({}_{1}\)\(\rightarrow\)\({}^{4}\)A\({}_{2}\) transition due to the partial relaxation of the spin multiplicity rule [6,8]. All these transitions are schematized in the diagram of Figure 4d. By monitoring the luminescence at the maximum of the intra-ionic Cr\({}^{3+}\) lines, it was verified that the main populating mechanisms in the studied spectral range correspond to the spin-allowed \({}^{4}\)A\({}_{2}\)\(\rightarrow\)\({}^{4}\)T\({}_{2}\), \({}^{4}\)T\({}_{1}\) transitions, giving rise to asymmetric broad excitation bands (FWHM of ca. 450 meV) with maxima around 3.1 eV and 2.2 eV. The asymmetrical band shape is in accordance with the splitting of the excited states into three Kramers doublets. However, the FWHM is wider than the ones reported for a single Cr\({}^{3+}\) center, which is about 250 meV [2]. Therefore, the broad asymmetric excitation bands likely correspond to a spectral overlap of several excitation bands due to multiple optical centers of Cr\({}^{3+}\). Besides the R-lines, a set of other narrow lines overlapped on the onset of the broad excitation bands were identified. In particular, the excitation lines corresponding to the parity and spin-forbidden \({}^{4}\)A\({}_{2}\)\(\rightarrow\)\({}^{2}\)T\({}_{1}\) R\({}^{\prime}\)-lines were seen to be a mirror image (without Stokes shift) of the ones observed in the PL spectra. Moreover, additional sharp lines are seen 50 meV apart in the blue region of the electromagnetic spectrum, at 2.66 eV and 2.61 eV, respectively, assigned to the \({}^{4}\)A\({}_{2}\)\(\rightarrow\)\({}^{2}\)T\({}_{2}\) transitions (B-lines).