## 3 Results  
The elemental composition of the NEG thin films was measured by RBS at different positions, showing that the composition is very uniform. From the RBS spectrum, the elemental concentrations of TiZrV were 24 atomic % Ti, 50 atomic % Zr, 26 atomic % V. The elemental concentrations of ZrVHf were 37 atomic % Zr, 42 atomic % V, 21 atomic % Hf. Figure 1 shows SRPES spectra of the C 1s core-level peak of different NEG films recorded after heating at various temperatures. The differentiation of the C 1s spectra gave several component peaks representing carbon in various chemical states. After degassing at 80 \({}^{\circ}\)C, the spectra of all films showed peaks of adventitious carbon contamination with C-C/C-H, O-C-O and O-C=O at binding energies of 284.8 eV, 286 eV and 288.5 eV, respectively [24]. For the Zr film (Figure 1a) at temperatures of 300 \({}^{\circ}\)C, there was a chemical shift to a lower binding energy of 281.1 eV, typical for zirconium carbide [25]. This suggests that the surface carbon partly desorbs and is partly transformed to zirconium carbide. For ZrVHf film (Figure 1b), the chemical shift to metal carbide appeared after heating at 200 \({}^{\circ}\)C, a temperature much lower than that of the Zr film. After heating at 200 \({}^{\circ}\)C, the C 1s spectra showed a detectable quantity of V-C at 282.2 eV [25] and Zr-C at 281.3 eV. After heating at 250 \({}^{\circ}\)C, the metal carbide peak shifted to a lower binding energy of 281.1 eV, typical for Zr-C, with still a possible small amount of V-C at 282.2 eV. After heating at 300 \({}^{\circ}\)C, the Zr-C peak partly shifted to 280.8 eV, typical for Hf-C [25]. The above shifting of the metal carbide peaks suggests that as the heating temperature increases from 200 \({}^{\circ}\)C to 300 \({}^{\circ}\)C, the transformation of metal-C bond follows the order of V-C, Zr-C, Hf-C.  
For TiZrV film (Figure 1c), the chemical shift from adventitious carbon to metal carbide at 282.1 eV, typical for V-C, appeared at temperatures as low as 120 \({}^{\circ}\)C. After heating at 180 \({}^{\circ}\)C, the intensity of carbon peak reduced and the V-C peak shifted to 281.5 eV, typical for Ti-C [25], suggesting that the surface carbon desorbs and vanadium carbide is transformed to titanium carbide. After heating at 200 \({}^{\circ}\)C, the surface carbon peak disappeared, and the carbide peak shifted to 281.1 eV, typical for Zr-C. After heating at temperatures above 250 \({}^{\circ}\)C, the Zr-C peak disappeared. The above shifting of the metal carbide peaks suggests that as the heating temperature increases from 120 \({}^{\circ}\)C to 200 \({}^{\circ}\)C, the transformation of metal-C bond follows the order of V-C, Ti-C, Zr-C. After heating at temperatures above 250 \({}^{\circ}\)C, the carbon bonded to zirconium diffused from the surface into the bulk [26]. Figure 2 shows SRPES spectra of the O 1s core-level peak of different NEG films recorded after heating at various temperatures. The deconvolution of O 1 s spectra resulted in the observation of O-C bond at 533.2 eV, -OH bond at 532.0 eV and lattice-O bonds ranging from 531.0 eV to 530.0 eV [27]. Figure 1: SRPES spectra of the C 1s core-level peaks of (**a**) Zr film, (**b**) ZrVHf film and (**c**) TiZrV film recorded after heating at various temperatures. Figure 2: SRPES spectra of the O 1s core-level peaks of (**a**) Zr film, (**b**) ZrVHf film and (**c**) TiZrV film recorded after heating at various temperatures. For Zr film (Figure 2a), as the heating temperature increased from 80 \({}^{\circ}\)C to 250 \({}^{\circ}\)C, the intensity of the O-C and -OH group gradually reduced. After heating at 300 \({}^{\circ}\)C, the intensity of the lattice-O peak at 531.0 eV corresponding to O in ZrO\({}_{2}\)[28] partly reduced.