## 4 Discussion
### Impact of Plasma Post-Processing  
As mentioned in Section 3.2, cerium was not removed by plasma post-processing; its content was even slightly increased. This indicates that the detected cerium was not surface-adherent, but implanted within a subsurface zone during polishing. Considering that the information depth of XPS is approximately 10 nm and that the mean thickness of hydrated silica layers is in this order of magnitude (1 to 20 nm [6]), the cerium content measured via XPS was most likely embedded in such a silica layer which was formed in the course of the polishing process. Apparently, this layer was not removed by the short-term plasma treatment due to its chemical stability. The slight increase in cerium content rather indicates a plasma-induced uncovering of this element by the removal of surface-adherent carbonaceous contaminants as discussed in more detail below. It can finally be stated that even though cerium is known to be a precursor for laser damage [12], its impact on the observed increase in LIDT can be neglected in the present case. In contrast, the correlation of the LIDT and the near-surface carbon content is obvious; as shown in Figure 7, the LIDT increases with decreasing carbon content. The impact of the presence of carbon on surface absorption and the resulting LIDT was already discussed in Section 4.1. The removal of carbon by the plasma thus allows reducing the effects and mechanisms mentioned there. Plasmas are well known to be suitable for the removal of surface-adherent hydrocarbons (C\({}_{x}\)H\({}_{y}\), where x < y) or other carbonaceous compounds. For instance,  
Figure 7: Correlation of the carbon content and the laser-induced damage threshold (LIDT) of polished zinc crown glass samples with and without plasma post-processing including general trend (dotted line). decomposition of hydrocarbons can be initiated by atomic oxygen (O) which is provided via dissociation within the plasma volume by collisions of oxygen molecules (O\({}_{2}\)) with free electrons. According to  
\[2 CH_{2}(s)+6O(p)\to 2 CO_{2}(g)+2H_{2}O(g) \tag{2}\]  
the resulting volatile by-products are then gaseous carbon dioxide (CO\({}_{2}\)) and water (H\({}_{2}\)O). This process is very efficient when working with \"cold\" dielectric barrier discharges [52] and when adding marginal fractions of oxygen (i.e., approximately 1 to 2%) to argon as carrier gas [30; 31]. In the present case, the plasma is driven at atmospheric pressure without any housing. Oxygen from the surrounding air is thus taken in by the plasma, subsequently mixed with the used argon and finally subject to the electric discharge within the discharge gap between the high voltage electrode and the sample surface. Apart from such pure chemical plasma-induced reaction of oxygen species with contaminants, further effects could contribute to the removal of carbon and the cleaning process, respectively. _[PERSON]_ and co-workers have reported on the increase in LIDT of crown glass due to laser cleaning at low fluence below the ablation threshold. The authors attributed this effect to a removal of absorbers from the glass surface by irradiation-induced desorption [53]. This mechanism could also play a role in the present case. Even though the plasma fluence is quite low, i.e., \(F_{\text{plasma}}=0.54\) J/cm\({}^{2}\)[39; 40], the plasma used in this work is ignited directly on the glass surface and thus provides a considerable amount of ultraviolet irradiation which might induce desorption. Another possible underlying mechanism, the emission of electrons or ions from the surface and the resulting alteration of electrical forces within the surface which bind the contaminants, was reported by _[PERSON]_ and _[PERSON]_[54]. This effect finally leads to a removal of contaminants. As a result of direct plasma ignition on the sample surface, the impact of charged particles on the surface is very high and might support the removal of carbon as observed in this work.