## 3 Results and Discussion
### Discussion and Outlook  
Alkali earth halides--CaF\({}_{2}\) more frequently than BaF\({}_{2}\)--have been exposed to intense ionizing and non-ionizing radiation many times. The main motivation of these studies lies in the study of damage to optical components and the manufacture of micro/nanostructures. Therefore, we may compare the characteristics of periodic nanostructures observed in our irradiation experiment with the surface damage patterns created under different irradiation conditions. Grating-like structures (LIPSS) appeared on CaF\({}_{2}\) and BaF\({}_{2}\) surfaces exposed to picosecond [26] and femtosecond [27; 28; 29] long-wavelength laser pulses, respectively. In both cases, the periodicity of the ripples was comparable to the laser wavelength and the LIPSS grating vectors depended on the direction of the polarization vector of laser radiation. This contrasts with our finding, where it is the orientation of crystallographic planes which is related to the grating-vector direction. It seems that the crystallographic planes played an important role in the damage to CaF\({}_{2}\) induced by nanosecond pulses of UV laser radiation [30; 31]. The damage pattern shows linear edges oriented along the crystallographic planes. However, their spacing is at least by one order of magnitude greater than in the case of XUV-CDL irradiation and they often cross each other at certain angles while the lines of the XUV-LIPSS reported here are dominantly parallel. Even nanoparticles emitted from BaF\({}_{2}\) irradiated by the nanosecond laser radiation often show a calcite crystal habitus and sharp edges [32]. This all indicates that laser-induced cleavage of alkali-earth halide crystals along the crystallographic planes can be observed in both the cases but with the long-wavelength laser radiation they have not been found to be as regular and uniform as observed on the XUV-CDL-illuminated surfaces in the present study. Both long- and the short-wavelength radiation may create various defects in the crystal lattice of insulators [33] (similar effects are reported also in wide-gap semiconductors [34]) resulting in irreversible changes of illuminated surfaces. In CaF\({}_{2}\), the formation of defects leading to surface metallization was induced by nanosecond pulses of 193-nm [35] and 157-nm [36] excimer laser radiation. However, the calcium-rich nanostructures show an oval shape not a linear one. CaF\({}_{2}\) has also been irradiated at a wavelength of 46.9 nm [37] by a similar XUV-CDL source as utilized in the present study. However, only the ablation thresholds and the rates were measured. A detailed characterization of ablated areas has not been performed. As to ionizing radiation, the defect formation was induced in BaF  
Figure 5: AFM images of ablation craters (**a**,**c**) created at a different orientation of crystallographic planes with respect to the tangential plane of the focusing mirror—positions A and B in Figure 4; (**b**,**d**) are highly-resolved AFM images measured inside these two craters. and CaF\({}_{2}\) by synchrotron radiation [38] and 850-eV [39] electrons, respectively. The results are similar to those obtained with intense UV radiation. It is clearly visible that the periodic nanostructures observed in the XUV-CDL irradiation experiment differ from the morphology of all the damage patterns observed on alkali-earth-halide surfaces exposed to radiation of various kinds in numerous studies reported earlier [26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. We should keep in mind that an attenuation length of 46.9-nm XUV-CDL radiation in BaF\({}_{2}\) is very short, i.e., only about 10 nm [40; 41]. Thus, energy density gradients are enormous in the near-surface region of the irradiated crystal. Thus, unique thermo-mechanical processes (and their interplays with other phenomena) may occur in a very thin near-surface layer under these irradiation conditions.