# Extended Photoionization Cross Section Calculations for C III
## 2 Computation Methods and Results
### Photoionization and Photoabsorption Cross Sections
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**J = 1\({}^{\rm o}\) Levels**} & \multicolumn{2}{c}{_E (cm\({}^{-1}\))_**} & \multicolumn{2}{c}{_E (cm\({}^{-1}\))_**} & \multicolumn{2}{c}{_E (cm\({}^{-1}\))_**} & \multicolumn{2}{c}{_E (cm\({}^{-1}\))_**} & \\  & **Relative to** & **Relative to** & **Relative to** & **Relative to** & **Relative to** & **\%** \\  & **1s\({}^{2}\)2s** & **BPRM** & **RMPS** & & **BPRM** & **NIST** & \\ \hline
2P\({}_{1/2}\)3P\({}^{\rm o}\)\({}^{\rm e}_{0}\) & \(-\)55,379 & 2.8626 & \(-\)55,982 & 2.8551 & 330,209.6 & 329,685.38 & 1.59\({}^{-03}\) \\ \hline
2s5s \({}^{1}\)S\({}^{\rm o}_{0}\) & \(-\)47,302.3 & 4.5692 & \(-\)47,396 & 4.5649 & 338,287.1 & 338,514.33 & \(-\)6.71\({}^{-04}\) \\ \hline
2P\({}_{3/2}\)3P\({}^{\rm o}\)\({}^{\rm e}_{0}\) & \(-\)39,774.1 & 3.0666 & \(-\)40,492 & 3.0557 & 345,815.3 & 345,095.43 & 2.08\({}^{-03}\) \\ \hline
2s6s \({}^{1}\)S\({}^{\rm o}_{0}\) & \(-\)30,404.6 & 5.6992 & \(-\)30,523 & 5.6884 & 355,184.8 & - & \\ \hline
2s7s \({}^{1}\)S\({}^{\rm o}_{0}\) & \(-\)22,187.3 & 6.6716 & \(-\)22,232 & 6.6653 & 363,402.1 & - & \\ \hline
2s8s \({}^{1}\)S\({}^{\rm o}_{0}\) & \(-\)17,045.1 & 7.6611 & \(-\)16,852 & 7.6557 & 368,544.3 & - & \\ \hline
2s9s \({}^{1}\)S\({}^{\rm o}_{0}\) & \(-\)13,185.5 & 8.6544 & \(-\)13,203 & 8.6494 & 372,403.9 & - & \\ \hline
2s10s \({}^{1}\)S\({}^{\rm o}_{0}\) & \(-\)10,606.7 & 9.6444 & \(-\)10,619 & 9.6444 & 374,982.7 & & \\ \hline Last bound & & & & & & \\
2P\({}_{1/2}\)4P \({}^{3}\)P\({}^{\rm o}_{0}\) & \(-\)1206.1 & 3.8582 & \(-\)1165 & 3.8584 & 384,383.3 & 384,345.00 & 9.96\({}^{-05}\) \\ \hline
2s\({}_{1/2}\) & 0 & \(\infty\) & 0 & \(\infty\) & 385,589.4 & 386,241.0 & 1.68\({}^{-03}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._Figure 6 presents the photoionization spectra from 2s5s (\({}^{1}\)S\({}^{e}\)) and 2p3p (\({}^{1}\)S\({}^{e}\)) of CIII, the dominant contribution of 2\(p\)nd \({}^{1}\)P\({}_{1}\)\({}^{\circ}\) configurations, and the clearly visible 2\(p\)ns (\({}^{1}\)Po) autoionizing series. The cross section of the 2\(p_{3/2}\)5s (\({}^{1}\)Po\({}_{1}\)) resonances is substantially reduced in the 2\(p_{3/2}\)3\(p\)\({}^{1}\)S\({}^{e}\) photoionization spectrum, while other 2\(p\)nd (\({}^{1}\)Po\({}_{1}\)) and 2\(p\)ns (\({}^{1}\)Po\({}_{1}\)) remain strong. The ratios of the 2\(p_{3/2}\)_n_ (\(n\geq 5\)) \(d\) (\({}^{1}\)Po\({}_{1}\)) peak photoionization cross sections between 2\(p\)3\(p\) (\({}^{1}\)S\({}^{e}\)) and 2s5s(\({}^{1}\)Se) spectra are approximately 2.36, which is close to the ratio of 2.6 between the 2\(p\)3\(p\) components in the 2\(p\)3\(p\)\({}^{1}\)S and 2\(s\)5s \({}^{1}\)S initial state [41]. Looking at the cross section ratios as functions of photon energy, we note that, due to the interchannel coupling which falls off with increasing energy, the features of all \(np\) and \(nd\) photoionization cross sections are independent of \(l\) for \(l>0\) and \(\sigma_{nl}\) (\(\omega\)) \(\sim\)1/\(\omega^{9/2}\), in agreement with ref. [42]. Figure 4: CIII photoionization cross sections (Mb) out of the 1s\({}^{2}\)2s\({}^{2}\) ground state, as a function of photon energy (Ry) below the 2\(p\) threshold. Figure 3: Spectral features of photoionization on the ground level in C III. The vertical dotted lines represent the C IV 2s \({}^{2}\)S\({}_{1/2}\), 2p \({}^{2}\)P\({}_{1/2,3/2}\), 3s \({}^{2}\)S\({}_{1/2}\), and 3p \({}^{2}\)P\({}_{1/2,3/2}\) thresholds, respectively. Photoionization cross sections (Mb) are plotted versus photon energy (Ry). Compared to the _ns_, the \(\sigma_{n1}/\sigma_{(n-1)1}\) ratios remain constant with energy, while \(\sigma_{\text{ns}}/\sigma_{(n-1)s}\) ratios decrease with energy and become constant with higher energy. For higher 2_ns_\({}^{1}\)S bound excited states with small 2_pnp_ mixing, the photoionization is dominated by 2s-2p core excitation, followed by the shake-up of the outer _ns_ electron, similar to the two-electron excitation of the core.