## 5 Discussion: Mechanism of Surface Hydroxylation of Fused Silica
The whole action process can be expressed as follows:  
\[Si_{2}\text{O}_{5}{{}^{-}}+H^{+}\leftrightarrow HO\cdot Si_{2}O_{4} \tag{3}\]  
\[Si_{2}\text{O}_{3}{{}^{+}}+OH^{-}\leftrightarrow Si_{2}\text{O}_{3}\cdot HO \tag{4}\]  
Generally speaking, the more unsaturated the bonding structure, the stronger its adsorption capacity for water molecules, thus the stronger its surface hydroxylation ability, and correspondingly, the higher the number of OH- groups formed on its surface. Due to the limitation of the bonding ability of Si atoms, the number of OH- groups of a single Si atom after hydroxylation is not more than 3. [PERSON] et al. [25] found that the average area density of OH- groups on the traditional polished fused silica surface after water cleaning is 4-6/nm\({}^{2}\). After the surface hydroxylation process, the binding force between surface atoms will be relatively weakened due to the introduction of OH- groups, and the binding energy of surface atoms after surface hydroxylation can be expressed as [26]:  
\[E_{as}=E_{bs}-(1/2)kTn_{OH} \tag{5}\]  
Figure 5: Schematic diagram of the unsaturated structure on the silica surface. where \(E_{\text{as}}\) is the binding energy of surface atoms after surface hydroxylation; \(E_{\text{bs}}\) is the binding energy of surface atoms before surface hydroxylation; \(n_{\text{OH}}\) is the number of OH-groups generated by a single Si atom (\(n_{\text{OH}}=1\), \(2\), \(3\)); \(k\) is the Boltzmann constant, and \(T\) is the ambient temperature. The more OH- groups adsorbed during the surface hydroxylation process, the more severe the weakening of the atomic binding energy of the surface layer, resulting in a reduction in surface mechanical strength. The hydroxylation process on the silica surface destroys the original spatial network structure of the silica bulk, thereby reducing its mechanical strength [16]. The strong hydroxyl characteristic peak of fused silica samples treated by HF acid etching is mainly due to the acid etching process occurring in a water environment. Once the freshly etched surface is exposed, it is easy to have surface hydroxylation with the surrounding water molecules, resulting in the formation of hydroxyl on the silica surface. After the removal of 1000 nm by ion sputtering, the hydroxylation layer on the surface was removed, resulting in a decrease in the characteristic hydroxyl peak intensity. When the ion-sputtered surface is completely immersed in deionized water, the chemical activity of surface atoms is enhanced by ion sputtering, which improves the probability of reaction between the unsaturated structure of the silica surface and water molecules, resulting in a further increase in surface hydroxyl content. This is the reason for the enhancement of the characteristic hydroxyl peak after immersion in deionized water. In addition, the characteristic hydroxyl peak intensity of the IBS surface after immersion in deionized water for 30 min is stronger than that of HF acid etching for 120 min, indicating that ion sputtering increases the unsaturated structure, enhances the activity of Si and O, and thus makes the surface hydroxylation reaction easier to occur. Due to the corrosive effect of OH- ions on the silica surface, chemical adsorption will occur and destroy the original spatial network structure of the silica bulk, resulting in a reduction in mechanical strength and a decrease in laser damage resistance performance. Therefore, the ion sputtering surface should be strictly avoided from contact with the water environment to avoid a decrease in the laser damage threshold caused by surface hydroxylation.