## 3 Results and Discussion  
It is well known that atmospheric pressure plasmas are efficient tools for cleaning glass surfaces by the removal of organic contaminations as demonstrated by various authors in the past [24,42, 43, 44, 45]. The high potential of this technique was also observed in the present case. As shown in Figure 3, even persistent fingermarks can be removed from polished lens surfaces. Here, a quite long-term plasma treatment as described in Section 2.2 was performed for 5 min in order to visualise its remarkable impact on such severe surface contamination. The effect of plasma-induced cleaning is due to several interacting plasma-chemical and plasma-physical mechanisms. First, the presence of surrounding air allows a chemical cleaning by the decomposition of surface-adherent hydrocarbons. In an atmospheric pressure plasma operated as in the present case, surrounding air is ingested and mixes with the actual plasma process gas, argon, which thus practically acts as carrier gas. Subsequently, molecular oxygen is dissociated to atomic oxygen (e.g., by electron impact-induced dissociation) as observed via plasma emission spectroscopy. Due to the discharge geometry, this occurs close to the sample surface. Once atomic hydrogen gets in contact with surface contaminants, it is capable of decomposing hydrocarbons to gaseous, volatile carbon dioxide and water. Here, an intermediate species, ozone (O\({}_{3}\)), is also of essential importance. Another process can also induce such decomposition: The high-energetic UV-irradiation, which is emitted by the plasma directly on the sample surface, gives rise to a photo-fragmentation of oxygen molecules. At the same time, surface-adherent hydrocarbons are decomposed by the irradiation. The plasma species formed in that way, i.e., atomic oxygen, carbon and hydrogen as well as molecule fragments and ozone finally form volatile gaseous water and carbon dioxide. At last, pure physical  
Figure 3: Visualisation of the impact of plasma treatment on a polished lens surface with fingermarks (light microscopic image, magnification 20x); left: before plasma treatment, right: after plasma treatment for 5 min. bombardment of the surface with argon ions from the plasma may further contribute to surface cleaning by sputtering. However, the mean kinetic energy of ions in DBD plasmas is normally rather negligible as represented by the moderate gas temperature [46]. In contrast, excited state argon is capable of transferring a notable amount of energy (e.g., 11.5 eV for metastable argon [47]) to surface-adherent contaminants. The number of such states is further considerably higher than the number of argon ions. The energy provided by de-excitation processes of excited argon states could thus represent an essential additional contribution for the cleaning process. In any case, the observed surface cleaning is obviously due to a plasma-induced removal of hydrocarbons or other carbonaceous surface-adherent contaminants. Apart from removal, plasma is capable of modifying carbonaceous surface-adherent contaminants by altering the percentage of involved carbon-based bonds as measured via X-ray photoelectron spectroscopy (XPS) and presented in previous work [38]. Such removal or modification of carbonaceous bonds can easily be detected by a decrease in water contact angle [38]. In the present case, the water contact angle was reduced from approximately 19\({}^{\circ}\) before plasma treatment to \(<4^{\circ}\) after plasma treatment for 10 s. This decrease corresponds to a plasma-induced increase in surface energy of the treated glass samples which directly impacts the wetting characteristics of cement applied to the glass surface as shown in Figure 4.  
It turns out that the contact angle of the used cement is decreased by a factor of about 1.3 (from 30.7\({}^{\circ}\) to 23.7\({}^{\circ}\)) after plasma treatment for merely 10 s. This corresponds to an improved wetting and thus an enhanced adhesion of the cement at the glass surface.