## 3 Results and Discussion
### Chemical Composition of the Surface and Water Bath  
Changes in chemical composition of polymer surface after underwater laser treatment were recorded by ARXPS and are given in Table 2. Atomic concentrations of O (35.2 at. %) and C (64.8 at. %) in pristine PET corresponds well to polymer stoichiometry with theoretical concentrations of O (33.0 at. %) and C (67.0 at. %), which are often shifted in favor of carbon due to hydrocarbon impurities being adsorbed on the surfaces of the polar polymeric materials from air [16]. Advantage of angle-resolved XPS analysis lies in the possibility to sensitively distinguish between the element concentration in thick surface polymer layer (typically 10 atomic layers, electron collection angle of 0\({}^{\circ}\)) and ultrathin surface shell (typically 1 to 2 atomic layers, electron collection angle of 81\({}^{\circ}\)). Thus, samples of pristine PET typically exhibit increase of carbon concentration when analyzed under 81\({}^{\circ}\), especially due to absorbed hydrocarbons in mono-atomic layer (see Table 2). Apparently, when increasing applied laser fluence the discrepancy between \"bulk\" and \"very surface\" concentration of C and O gradually disappears. This phenomenon most likely originates from (i) massive polymer ablation (absorbed hydrocarbons are removed), (ii) dramatic photochemical decomposition of macromolecular structure which unifies the chemical composition of the surface in a deeper profile [4]. Both of these contributions being affected by considerable changes in surface morphology, especially regarding data obtained under 81\({}^{\circ}\), where the analytical information \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Laser Fluence [m]-cm\({}^{-2}\)]**} & \multicolumn{6}{c}{**Applied Voltage [kV]**} \\ \cline{2-7}  & **0** & **19** & **21** & **23** & **25** & **27** & **29** \\ \hline
**0** & 0.7 & - & - & - & - & - & - \\
**8** & - & 25.2 & 23.4 & 22.6 & 21.8 & 23.3 & 25.3 \\
**12** & - & 28.3 & 28.7 & 31.6 & 30.3 & 28.8 & 29.0 \\
**16** & - & 31.2 & 30.6 & 33.6 & 32.3 & 31.9 & 31.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Surface roughness \(R_{\text{a}}\) (in nm) of underwater laser treated samples at laser fluencies of 0, 8, 12, and 16 mJ\(\cdot\)cm\({}^{-2}\) and different values of applied voltages. Presented \(R_{\text{a}}\) stands for the average value from seven measurements performed on each sample (according to Figure 2); variation of \(R_{\text{a}}\) value did not exceed 5 %. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Laser Fluence [m]-cm\({}^{-2}\)]**} & **Angle** & **Voltage** & \multicolumn{2}{c}{**Concentration**} \\  & [\({}^{\circ}\)] & **[kV]** & **[at. \%]** \\ \cline{3-6}  & & & **C** & **O** \\ \hline
**0** & 0 & 81 & 0 & 35.2 & 64.8 \\ \hline
**8** & 0 & 81 & 19 & 75.5 & 24.5 \\ \cline{2-6}  & 0 & 81 & 83.3 & 16.7 \\ \cline{2-6}  & 0 & 25 & 76.3 & 23.7 \\  & 81 & 25 & 81.5 & 18.5 \\ \hline
**12** & 0 & 19 & 79.7 & 20.3 \\  & 81 & 19 & 83.8 & 16.2 \\ \cline{2-6}  & 0 & 25 & 79.6 & 20.4 \\  & 81 & & 82.3 & 17.7 \\ \hline
**16** & 0 & 19 & 81.2 & 18.8 \\ \cline{2-6}  & 81 & 25 & 80.9 & 19.1 \\ \cline{2-6}  & 81 & 25 & 82.1 & 17.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Atomic concentrations of C(1s) and O(1s) in pristine and underwater treated PET measured by ARXPS under electron take-off angles of 0 and 81\({}^{\circ}\) for different operating voltages. comes predominantly from the ridges of induced nanostructure. Besides this, these changes are obviously accompanied by considerable graphitization of polymer surface [1] which contributes to increasing concentration of carbon with increasing applied fluence. This finding is similar to that of [PERSON] et al. [15], who identified the hydrolysis reaction responsible for decrease of laser threshold during excimer laser processing of polyesters which increases the efficiency at low applied fluencies.