## 4 Discussion
### Formation of the Oxide Layer and Debris Deposition  
The surface changes induced by femtosecond irradiation on Ni/Al multilayers showed a delicate dependence on laser parameters such as pulse duration, pulse energy, fluence, and repetition rate. It is well known that after femtosecond laser irradiation in air, the treated sample is covered by debris originating from redeposited matter that mainly occurs in the shape of oxidic particle agglomerations. The quantity and extension of debris found in the vicinity of the ablated trenches displayed in the SEM micrographs (Figure 2a-c) deviates  
Figure 6: Normalized EELS intensity mapping of the elements (**a**) Al, (**b**) Ni, and (**c**) O in the 100 fs_1 kHz sample. A significant amount of Ni can be found in the first 50 nm of the Al layers. The oxide region contains mainly aluminum. Arrows in (**b**) indicate how far the Ni signal was observed in the Al layers. Red crosses indicate the positions of EELS composition measurements. significantly between 1 and 5 kHz. However, between the different pulse durations (100 and 150 fs) at 1 kHz, only a slight change in surface and cut edge morphology was visible. The overall amount of debris formation could be expected to be of similar quantity since spot size, pulse energy, and ablation depths inducing a complete cutting of foils range in similar levels between the different samples. Subsequently, variations in local accumulation point towards alteration in the underlying ablation mechanisms inducing deviating expulsion kinetics and trajectories. Although the ablation thresholds for Ni, Al, and their intermetallic compounds [33; 34; 35] are within similar orders of magnitude, previous research [30] suggests layer-by-layer ablation of the individual Ni and Al double layers. Applying this consideration on the conducted single pass processing of Ni/Al at either 1 kHz or 5 kHz, a deviation in the development of the advancing cut edge morphology can be suggested. Terraces formed of Ni/Al double layers with lower (1 kHz) or higher (5 kHz) front inclination originate from the difference in pulse overlap (as schematically displayed in Figure 7). USP irradiation of Ni in the applied moderate fluence regime of 1.1-1.5 J/cm\({}^{2}\) induces ablation via phase explosion of the overheated material, which is characterized by elevated kinetics of expulsed matter also involving mobilization of liquid droplets. Above the threshold, the plume is predominantly composed of clusters, and the quantity of ejected matter is determined by the critical volume energy density delivered by the laser pulse [36; 37]. Clear indications of phase explosion ablation on our samples are the shock-like solidified melt structures of the HAZ (Figure 4d-f). The onset of matter expulsion generally faces in the opposite direction of the solid surface and, as a result, alteration in the cutting front affects the trajectory. This causes the matter to be predominately transported out of the cutting trench at 1 kHz, while at 5 kHz, the expelled matter tends to travel along the trench instead (Figure 7). Consequently, both 100 fs_1 kHz and 150 fs_1 kHz exhibit expanded debris areas, where the loosely attaching porous morphology points towards considerable cooling of the oxide particles before redeposition, indicating an extended flight path. The thicker internal oxide layer and reduced external debris redeposition for 150 fs_5 kHz suggests that the ablation trajectory is leading to agglomeration within the trench. This is indicated by a pile-up at the cut edges, rather than ejection. In parallel to the shrinking of trench width, deterioration of the cut edge line can be observed, which is related to inhomogeneous debris/oxide accumulation. A similar edge morphology can be found on the 100 fs_1 kHz sample in combination with a lesser debris extension compared to the 150 fs_1 kHz sample. Here, the role of the small difference in pulse duration is of lesser importance than the difference in fluence applied.