## 3 Results and Discussion
### Chemical Analysis  
The surface chemical circumstance is one of the key factors affecting the SEY of the surfaces. Therefore, XPS analysis was adopted to inquire into the chemical states of the surface elements. On the basis of the survey, scan spectra of untreated Mo and laser processed Mo sample #4 (Figure 5) showed that Mo, C, N and O were the dominant elements on the sample surfaces. The element ratios (At%) of Mo, C, N and O for untreated Mo and laser processed Mo were 12.7%:29.2%:32.2%:25.9% and 10.2%:37.3%:27.1%:25.4%, respectively. After laser processing, the atom ratios of Mo and N decreased slightly by about 2.5% and 5.1%, respectively. While, the atom ratio of C increased by about 8.1%, which may have been caused by the introduction of increased carbon impurity with the increases of the surface area. The element ratios of the O element basically remained unchanged before and after laser processing. Spectra of the untreated Mo and laser treated Mo sample #4 with \(\delta_{\text{max}}\) less than one are shown in Figure 6. The doublet peaks of 235.59 eV and 232.45 eV in terms of Mo-3d3/2 and Mo-3d5/2 were attributed to the Mo-3d spin-orbit splitting, which was consistent with the values in other references [28; 29; 30]. Gaussian 70%-Lorentzian 30% was used for deconvolution of the XPS peak for Mo and oxygen. Figure 5: X-ray photoelectron spectroscopy (XPS) spectra of (**a**) untreated Mo and (**b**) laser treated Mo sample #4. Figure 6: XPS core-level spectra of molybdenum (Mo-3d) for (**a**) untreated Mo and (**b**) laser treated Mo. As discussed in refences [31; 32], screened refers to the components at low binding energy due to well screened final states in which the localized level becomes occupied by an itinerant conduction electron. Besides, the unscreened is referred to as the broader components at higher binding energy associated with unscreened final states, in which the localized level remains empty. This demonstrated that the chemical state of laser treated Mo sample surface was Mo6+. The Mo-3d spectrum of untreated Mo in Figure 6a indicated that the content of Mo metal, Mo4+ and Mo6+ were 48.54%, 30.53% and 20.93%, respectively. The parameters for the Mo-3d spectra curve fitted of untreated Mo were consistent with those of Reference [33]. The surface of untreated Mo metal showed a relatively high level of original Mo metal with a value of 48.54%, while that of the laser treated Mo sample #4 showed the chemical state was Mo6+ which was dominated by a spin-orbit doublet. The Mo 3d spectrum of untreated Mo metal surface indicated that the chemical states of Mo were much more complex and consisted of Mo metal, MoO\({}_{2}\) along with spin-orbit doublet of MoO\({}_{3}\). The chemical states of the Mo element of untreated Mo sample were complicated, which included Mo metal and molybdenum oxide at different valence states. After laser processing in the atmosphere, the molybdenum sample surface was completely oxidized with the chemical state of Mo6+. The characteristic peak positions for the O 1s state of untreated Mo and laser treated Mo sample #4 are presented in Figure 7. The O 1s spectrum of untreated Mo metal demonstrated that the percentages of O 1s-lattice oxide, O 1s-hydroxides and defect oxides, and O 1s-water and organic O were 31.71%, 50.06% and 18.23%, respectively. The concentrations of O 1s-lattice oxide, O 1s-hydroxides and defect oxides, and O 1s-water and organic O of laser treated Mo sample #4 were 73.67%, 24.92% and 1.41%, respectively. After laser processing, the percentage of O 1s-lattice oxide increased from 31.71% to 73.67%. Thus, stoichiometric formulation was formed on the surface of laser treated Mo metal. The metal oxides were almost always strong secondary emitters. However, many factors, such as surface oxidation states, surface morphology, surface roughness, etc., can affect the SEY of metal samples. In this paper, the laser ablation method was used to modify the surface morphology to form a low SEY surface (less than one).