## 3 Results and Discussion
However, the intensity distribution of the visible spot indicated the general condition of the intensity distribution of the focused laser spot. The third row was the corresponding damage pattern at each position. The white lines circled the melting position. To compare the three rows, the shape of the laser spot and the damage pattern were coincident. In each position, the melting occurred at the position under the laser irradiation with a higher intensity, which indicated a higher fluence. Figure 6: Details at the edge of the damaged area. In the damaged area, the melting traces shown in Figure 10 were expressed in several kinds of morphologies. The surface absorbed the laser energy, then was melted or evaporated and resolidified into nanoparticles, as shown in Figure 10a. Before cooled into a solid phase, the particles might converge and then be resolidified into small bulk, as shown in Figure 10b. Since the target was located vertical to the beam path and nearly vertical to the horizontal plane, most of the nanoparticles were drop-down because of gravity. Since the plasma expansion occurred near the surface of the target, there are still some particles left on the surface, which showed strong evidence that the surface behavior was  
Figure 8: Measurement of the damaged area on PMMA. Figure 5:0 Melting trace under different laser fluences. Figure 5:1 Whole image of the patterns at other positions: (**a**) Position at left of the patterns in Figure 7; (**b**) Position in the middle of the patterns in Figure 5:2  
drastic due to the irradiation of the laser. Phase explosion would occur during the interaction. Based on that, the thermal effect was strongly involved in the interaction. In future research, the mica sheet would be used below the target to catch the nanoparticles ejected from the surface to study the size distribution. In the ablation area, it was also found a large melting area around the impurities inset the copper surface, as shown in Figure 5:3 It was supposed to be caused by the heat accumulation around the interface of the impurity and copper. That would be another evidence to prove the thermal effect in the damage. Melting trace was detected in the ablation area induced by both single and multiple pulses of laser irradiation. Figure 12 showed the melted surface induced by 25, 12, 6, 3, 2, and 1 pulse of laser irradiation. The area showed in Figure 12 was part of the whole damage area which presented the level of the melting. The melting behavior got more and more intense with the increasing number of laser pulses. When the pulse number was above 3, the surface evaporation was quite clear. According to the analysis, there was no temperature accumulation in the multiple pulses of laser irradiation. Therefore, we could expect that there was also evaporation occurred in single pulse irradiation, which was too faint to be detected clearly under SEM. The estimated average fluence of a 46.9 nm laser that could cause evaporation was much lower than the fluence of optical lasers that could cause surface evaporation of copper [17]. It is generally known that the damage threshold of EUV laser is much lower than that of the laser with a longer wavelength, which due to the contribution of the photon with quite a high energy. However, the melting or evaporation threshold of the same material is more related to the fluence of the light, not the photon energy. Therefore, we supposed the quite low evaporation threshold of a 46.9 nm laser indicated that the high photon energy of a 46.9 nm laser promoted the  
Figure 5:4 Large area melted around the impurity: (**a**) Melting area around the impurity; (**b**) Boundary line between impurity and copper. Figure 5:5 Different types of melting traces: (**a**) Nanoparticles; (**b**) Melted bulk. thermal effect in the interaction process. More corresponding research is expected in the future to study the relationship between the high photon energy and the thermal effect.