## 3 Results and Discussion  
After the irradiation of the focused 46.9 nm laser with multiple pulses, ablation patterns shown in Figure 5 were detected by SEM. The ablation pattern induced by single-pulse was also detected, which indicated the laser fluence was above the damage threshold of copper. The melting occurred at a similar area of the ablation patterns and became more obvious with the increasing of the pulse number. The melting area was measured to be 770 \(\upmu\)m\({}^{2}\), 420 \(\upmu\)m\({}^{2}\), 270 \(\upmu\)m\({}^{2}\) and 120 \(\upmu\)m\({}^{2}\), with respect to the pulse number of 6, 3, 2 and 1. In the experiment, the interval time between every two pulses was longer than 30 s to avoid the superposition of the surface relaxation. The following laser pulse irradiated the surface was already melted by the last laser pulse. The increasing melting area with the added pulse number indicated that the characteristic of the melted surface was changed to promote the heat diffusion. Therefore, in the case of multiple laser pulses, the damaged area was melted again and again and presented quite a drastic surface behavior. From Figure 5, the melting part was only a small area in the whole damaged area. In most parts of the damage, the fluence of the laser was not high enough to trigger evident melting or evaporation behavior on the copper surface. However, the irradiation of the laser modified the surface into a different condition presenting a contrast with the color of the unabated area under SEM detection. Figure 4: Temperature variation on the different depth of the sample along the center axis. Figure 5: Whole view of the ablation patterns on copper-induced by multiple laser pulses. To study the difference between the damaged and undamaged areas, the edge of the damaged area was detected and shown in Figure 6. The fringes at the top of the damaged area were induced by the inhomogeneous energy distribution of the original laser spot and the diffraction effect occurred in the transmission. In the undamaged area, scratches induced by the surface polishing were observed clearly. Nonetheless, they did not exist in the damaged area, which proved the laser cleaning effect. To analyze the melting under different laser fluence, the focused laser spot at a different distance from the focus point was studied in Figure 7. We calculated the average fluence by the function as below:  
\[F=E_{pulse}/S_{damaged} \tag{2}\]  
where \(E_{pulse}\) was the pulse energy and \(S_{damaged}\) was the area of the damaged pattern. It was worth noting that \(S_{damaged}\) was measured on the damaged surface of polymethyl methacrylate (PMMA), not copper, since PMMA was more sensitive to the irradiation of the 46.9 nm laser and would help to present the whole shape of the focused 46.9 nm laser. Figure 8 showed the measurement of the damaged area on PMMA. The pattern was the whole view of the one showed at the right image in Figure 7. The damage pattern was detected by White light interferometer (WLI). The area was measured to be 0.04 mm\({}^{2}\). With the estimated energy of 100 \(\upmu\)J, the average fluence of the laser was calculated to be 250 mJ/cm\({}^{2}\). Figure 9a,b referred to the whole view of the patterns at left and in the middle of Figure 7. The area was measured to be 0.1 mm\({}^{2}\) and 0.06 mm\({}^{2}\), corresponding to the fluence of 100 mJ/cm\({}^{2}\) and 160 mJ/cm\({}^{2}\). The focused laser spot was caught by a Ce-doped Yttrium Aluminum Garnet (Ce: YAG) scintillator with the transformation from the invisible laser into visible light. The real shape of the focused laser at a different distance from the focus spot was presented in the middle row of Figure 7. The first row in the picture was the contour map obtained from the intensity of the visible spot. The color bar showed the scale of the relative intensity of the spot. The intensity of the laser spot and transformed visible spot were not in a linear relationship.