## 3 Results and Discussion
### Raman Analysis  
Surfaces of regions A1, B1, C1, D1, and E1 of the laser-processed Ti were analyzed using Raman microscopy to understand the influence of laser structuring on the formation of surface oxide phases on titanium alloy [9]. To bring out this influence clearly, the Raman spectrum of the polished CP titanium surface was also measured. When metals are processed by laser, there is a formation of oxides on the surface [49]. Raman is widely used to characterize transition metal oxide structures present on the surface because it is sensitive to metal oxide vibrations [54]. It is expected that there was a formation of titanium oxides on the surface after laser ablation in ambient conditions [9]. Titanium oxide phases are Raman active between 100-900 cm\({}^{-1}\) regions [55]. The rutile and anatase phases have four (A\({}_{1g}\), B\({}_{1g}\), B\({}_{2g}\), E\({}_{g}\)) and six (1A\({}_{1g}\), 2B\({}_{1g}\), 3E\({}_{g}\)) active modes with symmetries, respectively, which occur at a certain Raman shift [56]. The Raman spectra of the polished surface and the surfaces structured by a laser before immersion and after immersion in seawater are shown in Figure 5.  
It can be observed that the spectrum of the polished surface of the CP Ti has no notable peaks, while the spectra of the laser-structured surfaces have pronounced peaks [27]. The absence of notable peaks for polished CP Ti shows that its surface had only the naturally occurring thin oxide film. This corroborates the EDX findings that the polished CP Ti surface has elemental Ti as the main element (\(\sim\)99.1%). It can be noted that the spectra have peaks at various Raman shifts and with various intensities which depend on the oxide crystallinity formed during laser processing [55]. The Raman spectra of the surfaces of the regions before and after immersion in seawater have four peaks labelled P1, P2, P3, and P4, but, on the A1 region before immersion in seawater, peak P1 was not well resolved. Since the Raman peaks in Table 4 exhibit strong agreement with reference values for the rutile phase of the TiO\({}_{2}\)[56; 57], they have been allocated to various vibration modes based on their positions in the spectra. The most intense peaks, P3 and P4 (Figure 5), for the five regions which have been assigned to E\({}_{\rm g}\) and A\({}_{\rm 1g}\) respectively confirm that the oxide phase formed on the surfaces was mostly that of rutile [55; 58]. B\({}_{\rm 1g}\), E\({}_{\rm g}\), and A\({}_{\rm 1g}\) modes are associated with symmetric bending, symmetric stretching, and antisymmetric bending vibrations of O-Ti-O bonds, respectively [55]. Surfaces in the regions D1 and E1 before immersion in seawater have sharp peaks P3 and P4, with high intensities. This signifies that the amount of the oxide phases formed in these two regions is higher than in regions A1, B1 and C1 [55]. The higher intensities mean that the higher the laser fluence used, the higher the amount of the oxide phase formed on the surface. Through comparison of the peak positions of the regions before and after immersion in Table 4, it can be noted that the peak positions did not change much. Some peaks redshifted while others blueshifted and this could be attributed to the dimensional changes of the surface nanostructures on the laser-structured CP Ti material [22]. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Before Immersion**} & \multicolumn{6}{c}{**After Immersion**} \\ \hline
**Peaks** & **A1** & **B1** & **C1** & **D1** & **E1** & **A1** & **B1** & **C1** & **D1** & **E1** & **Assigned Mode** \\ \hline P1 & 153 & 157 & 154 & 143 & 154 & 169 & 151 & 167 & 151 & 154 & B\({}_{\rm 1g}\) \\ P2 & 242 & 242 & 242 & 259 & 266 & 246 & 264 & 254 & 272 & 261 & Multi-photon process \\ P3 & 417 & 418 & 419 & 422 & 423 & 420 & 420 & 417 & 415 & 416 & E\({}_{\rm g}\) \\ P4 & 608 & 605 & 609 & 608 & 605 & 597 & 609 & 603 & 604 & 606 & A\({}_{\rm 1g}\) \\ \hline \hline \end{tabular}
\end{table}