# Carbon Microstructures Synthesis in Low Temperature Plasma Generated by Microdischarges
## 3 Results
### Discharge Diagnostics by OES
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Transition (Upper Level Energy—Lower Level Energy [eV])** & **Position [nm]** \\ \hline \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{s}\) (13.328–11.548) & 696.5 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{s}\) (13.328–11.624) & 727.3 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{s}\) (13.302–11.624) & 738.4 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{s}\) (13.171–11.548) & 763.5 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{s}\) (13.238–11.723) & 794.8 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{s}\) (13.328–11.828) & 826.5 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{s}\) (13.153–11.723) & 866.8 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{s}\) (12.907–11.548) & 912.3 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{1/2})4\mathrm{s}\) (13.172–11.828) & 922.5 \\ \(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{p}\to\)\(3\mathrm{e}^{2}3\mathrm{p}^{5}(\mathrm{2P}^{\circ}{}_{3/2})4\mathrm{s}\) (12.907–11.624) & 965.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optical emission lines of argon used for the determination of excitation temperatures during the discharge in Ar/C\({}_{6}\)H\({}_{12}\) mixture [34]. Figure 8: Optical emission spectra taken from the tip of needle electrode and from plasma column at 2 mm from the needle tip, for various discharge currents and inter-electrode distances. The excitation energy of C\({}_{2}\) and CH is not very high (about 2.9 and 2.5 eV, respectively), but in order to obtain radicals from cyclohexane decomposition, much higher energy is required, and the chemical processes are much more complex. The most probable products, which could be obtained from cyclohexane decomposition, are ethane, methane and acetylene. For example, the emission of C\({}_{2}\) could be obtained by collision with electrons [37], as follows:  
\[\mathrm{C_{2}H_{2}+e\to C_{2}H+H+e,} \tag{4}\]  
\[\mathrm{C_{2}H+C_{2}H\to C_{2}+C_{2}H_{2},} \tag{5}\]  
or argon via an energy transfer from the metastable argon particles Ar*, as follows:  
\[\mathrm{C_{2}H_{2}+Ar^{*}\to C_{2}H+H+Ar,} \tag{6}\]  
\[\mathrm{C_{2}H+Ar^{*}\to C_{2}+H+Ar.} \tag{7}\]  
For CH emission, the possible reactions could be as follows:  
\[\mathrm{e+CH_{2}\to CH+H+e,} \tag{8}\]  
\[\mathrm{e+CH_{3}\to CH+2H+e,} \tag{9}\]  
where CH\({}_{2}\) and CH\({}_{3}\) could be obtained by the decomposition of CH\({}_{4}\)[38] or C\({}_{2}\)H\({}_{2}\) in electrical discharge [39]. [PERSON] et al. [40] measured the electron density in atmospheric pressure plasmas by comparing the ratio of the intensity of lines of argon 2p-1s transitions and interpreting them using a collisional-radiative model. The ratio of the intensity of lines of Ar (2p3 \(\to\) 1s4) at 738.4 nm and Ar (2p6 \(\to\) 1s5) at 763.5 nm is very sensitive to the variations of electron density in plasma in the range of 10\({}^{14}\)-10\({}^{16}\) cm\({}^{-3}\).