## 3 Results and Discussion
### Effective Ionization Coefficients  
The normalized effective ionization coefficient (\(\alpha-\eta\))/\(N\) of C\({}_{4}\)F\({}_{7}\)N, in a range of \(E\)/\(N\) from 750 to 1150 Td, at 20 \({}^{\circ}\)C was obtained as well. Figure 4 presents the value (\(\alpha-\eta\))/\(N\) as a function of \(E\)/\(N\) in C\({}_{4}\)F\({}_{7}\)N, and its comparisons with SF\({}_{6}\)[3], \(<\)C\({}_{4}\)F\({}_{8}\)[4], CF\({}_{3}\)I [7], C\({}_{4}\)F\({}_{7}\)N [13,14], and C\({}_{4}\)F\({}_{7}\)N/N\({}_{2}\) mixtures [18] are reported. Notably, the \(E\)/\(N\) of C\({}_{4}\)F\({}_{7}\)N was much greater than other kinds of gases for the same (\(\alpha-\eta\))/\(N\), which suggests that the insulation ability of C\({}_{4}\)F\({}_{7}\)N is much stronger. Once the C\({}_{4}\)F\({}_{7}\)N gas mixed with buffer gas N\({}_{2}\), the insulation performances of the mixtures were much weaker than pure C\({}_{4}\)F\({}_{7}\)N, since N\({}_{2}\) is an electrically neutral gas whose attachment coefficients is 0. Meanwhile,  
Figure 1: Values of \(\alpha\)/\(N\) and \(\eta\)/\(N\) as a function of \(E\)/\(N\) in C\({}_{4}\)F\({}_{7}\)N gas in the present work and their comparison to SF\({}_{6}\) gas.  
Figure 2: Comparison of total attachment cross-sections of C\({}_{4}\)F\({}_{7}\)N, SF\({}_{6}\), c-C\({}_{4}\)F\({}_{8}\), and CF\({}_{3}\)I.  
the variety of _(\(\alpha-\eta\))/N_ with _E_/_N_ showed a linear trend for all these gases nearby the normalized critical electric field (_E_/_N_)\({lim}\) (for \(\alpha-\eta=0\)). Moreover, compared to the data reported, our results of normalized effective coefficients in C\({}_{4}\)F\({}_{7}\)N were in good agreement with that of [PERSON] et al. [13] as well as [PERSON] [14]. Meanwhile, more values in the _E_/_N_ that varied from 750 to 1150 Td were obtained in the present work.  
According to the bond length values of the C\({}_{4}\)F\({}_{7}\)N molecule [21], the structure of the C\({}_{4}\)F\({}_{7}\)N molecule could be drawn as in Figure 5. A recent study [22] shows that the bonds of C-1 to C-2 and C-1 to C-3 have the smallest bond energy, which is 3.812 eV/atom. The second smallest bond energy is 4.556 eV/atom, which belongs from C-1 to F-1. The bond energy of S-F in SF\({}_{6}\) is 3.432 eV/atom, which is smaller than the smallest bond energy in C\({}_{4}\)F\({}_{7}\)N. It is well known that the smaller the bond energy is, the weaker the interaction between atoms will be. Since the C\({}_{4}\)F\({}_{7}\)N and SF\({}_{6}\) molecules have a strong ability to attach electrons, they can easily adsorb electrons and, hence, become negatively charged molecules. Then, these charged molecules could accelerate under an electric field, applied between two plate electrodes, and collide with other molecules. Thus, the collision may lead to the breaking of weak bonds and forming new particles such as F in SF\({}_{6}\) and CF\({}_{3}\) in C\({}_{4}\)F\({}_{7}\)N. For the same electric field, the bonds of SF\({}_{6}\) are easier to be broken than bonds C-1 to C-2 (or C-3) of C\({}_{4}\)F\({}_{7}\)N. Consequently, the new particles formed in the SF\({}_{6}\) gas, such as F, would further take part in the discharging process. However, the CF\({}_{3}\) formed in C\({}_{4}\)F\({}_{7}\)N more easily adorbs electrons than the F formed in SF\({}_{6}\), since there is more F in CF\({}_{3}\), and fluorine has strong electronegativity. Then, SF\({}_{6}\) is more likely to exhibit ionization characteristics, which could lead to the higher effective ionization coefficients of SF\({}_{6}\) than that of C\({}_{4}\)F\({}_{7}\)N for the same _E_/_N_.  
Figure 3: (**a**) Comparison of values of \(\alpha\)/N measured with the steady-state Townsend (SST) method in this work and the pulsed Townsend (PT) method [14]; (**b**) Comparison of values of \(\eta\)/N measured with SST method in this work and PT method [14].