## 3 Results and Discussion
### TPPE Mechanism and Its Relation to Activation Energy  
We propose a model of PE based on the tunnelling of electrons to the hydroxyl group radical (the formula of -Cu-OH\({}^{\bullet}\) is used to emphasize the radical), followed by Auger emission, and further, the activation energy of PE, which is provided by the heat produced when -Cu-OH\({}^{\bullet}\) attracts electrons from the base metal. [PERSON] [38] emphasized that a model for exoemission due to the chemisorption of electronegative gases on metals based on electron tunnelling followed by Auger emission is very promising. The reason why we adopt the electron tunnelling model is described as follows. A reactive hydroxyl radical (OH\({}^{\bullet}\)) can be generated in a Fenton-type reaction [4; 6]. The OH\({}^{\bullet}\) radical has a relatively great tendency to attract electrons to itself. This radical can be considered as one of the electronegative gases. The electron affinity of OH is reported to be 1.83 eV [39]. In Figure 10d, for sample A, we described that the \(N_  
The reason why we adopt the electron tunnelling model is described as follows. A reactive hydroxyl radical (OH\({}^{\bullet}\)) can be generated in a Fenton-type reaction [4,6]. The OH\({}^{\bullet}\) radical has a relatively great tendency to attract electrons to itself. This radical can be considered as one of the electronegative gases. The electron affinity of OH is reported to be 1.83 eV [39]. In Figure 10d, for sample A, we described that the \(N_{\rm Tmax}\)a values increase with increasing \(\Delta E_{\rm{Upl}}\) in the 1 st temperature increase process. This relationship is completely inconsistent with the observation that the total count of electrons emitted during the Up1 scan decreases with increasing activation energy (\(\Delta E_{\rm{Upl}}\)) for scratched Fe surfaces in Figure 10 of Ref. [37]. This suggests that the PE mechanism in the copper surfaces is more complicated. We think that the activation energies in the present copper surfaces cannot be explained by the \(ET\) taking place over the energy barrier of the overlayer. A key reaction step on the surface during the TPPE measurement is represented by the partial change in the O1s spectra from an OH component (Cu(OH)\({}_{2}\)) to an O\({}^{2-}\) component (Cu\({}_{2}\)O). Figure 13 shows a simple energy level diagram based upon electron tunnelling to adsorbed hydroxyl radicals followed by Auger emission. According to the review of atmospheric corrosion [25], the chemical reactions can be represented by Equations (1)-(4):  
\[{\rm Cu}+{\rm H}_{2}{\rm O}+(1/2){\rm O}_{2}{\rightarrow}{\rm Cu}({\rm OH})_{2} \tag{1}\]  
\[{\rm Cu}({\rm OH})_{2}{\rightarrow}{\rm H}_{2}{\rm O}+{\rm CuO} \tag{2}\]  
\[{\rm Cu}+{\rm CuO}{\rightarrow}{\rm Cu}_{2}{\rm O} \tag{3}\]  
\[{\rm Cu}_{2}{\rm O}+(1/2){\rm O}_{2}{\rightarrow}{\rm CuO} \tag{4}\]  
Figure 13: A simple energy level diagram based upon electron tunnelling to the trap (B) of the adsorbed hydroxyl radical (B) followed by Auger emission. The depth of the trap increases in the order (CH\({}_{3}\))\({}_{2}\)CHOH \(<\) C\({}_{2}\)H\({}_{5}\)OH \(<\) CH\({}_{3}\)OH \(<\) H\({}_{2}\)O for sample A and H\({}_{2}\)O \(<\) CH\({}_{3}\)OH \(<\) C\({}_{2}\)H\({}_{5}\)OH \(<\) (CH\({}_{3}\))\({}_{2}\)CHOH \(<\) C\({}_{3}\)H\({}_{7}\)OH for sample C; \(\varphi\) work function; \(E^{0}_{\rm A}\) (electron affinity for OH alone) = 1.83 eV; \(E^{\prime}_{\rm A}\), electron affinity of OH adsorbed on the Cu surface in the liquids; \(E_{\rm F}\), Fermi level. The two types of electrons for tunnelling to the surface hydroxyl group and for Auger emission from Fermi level, producing PE are represented by two letters of e.  
In the thin film on the sample surface during the TPPE measurement, referring to the XPS results shown in Figure 6, Equations (1)-(3) play an essential role in producing OH and O2-, where the Cu shows the route where the initial Cu atom is successively involved in these reactions. Equation (4) is not considered.