# Extended Photoionization Cross Section Calculations for C III
## 1 Introduction  
The spectrum of the carbon atom is important for various applications, from the interstellar medium and the atmosphere of stars to the production of carbon clusters such as fullerenes, the study of energy losses due to impurities in nuclear fusion, or the development of a number of laser-based diagnostic methods. The theoretical models and numerical methods differ from magneto-hydrodynamic models (in the chromosphere [1, 2]) to the modeling of the independent atom (in plasma of solar corona [3]). In the transition region the models become complex, including specific atomic processes. The emission and absorption lines of carbon ions are modeled according to the density and temperature parameters of the studied plasmas. For example, with increasing density, the dielectronic recombination role decreases [4, 5]. In modeling the emission lines observed in the solarcorona, this process is often ignored, considering that photoionization (PI) does not affect the diagnosis [6].  
CIII consists of 70 LS-coupled levels ranging up to \(n=9\) and \(l=2\), with fine-structure levels being available from the web-based compilation table of the National Institute of Standards and Technology (NIST) [7]. The interaction of carbon ions with electrons and photons was modeled in non-relativistic and relativistic approaches [8; 9; 10; 11]. Fourteen members, including C III, of the Be isoelectronic series [12] have been studied using the semi-relativistic Breit-Pauli R-matrix method [13]. Experimental and theoretical data were collected and critically evaluated by the Opacity Project team [14], [PERSON] and [PERSON] [15], [PERSON] and [PERSON] [16; 17], and [PERSON] et al. [18]. Recently, the theoretical models and numerical methods used in the opacity calculation have been re-examined [19] along with the hierarchy of approximations adopted in the R-matrix methodology to check the accuracy of data. The ab initio \(R\)-matrix Floquet method (RMF) was also used to include, non-perturbatively, radiative damping effects of electron scattering on CIII [20; 21; 22; 23]. Among the existing theoretical BPRM computations for electron excitation with C II and CIII, notable references exist in the literature [24; 25; 26; 27; 28]. Although a considerable amount of research has been conducted on carbon and its ions, there is still a need for extended sets of theoretical transition data.  
The objective of this work is to complete the existing reported radiative and autoionization datasets with new photoionization (PI) and photoabsorption (PA) cross sections for CIII. The spectral features of the photoionization on the ground level in CIII at photon energy below the 2p\({}_{3/2}\) ionization threshold, with complex resonances, have been obtained, and associated resonances were delineated. This study entails fine-structure atomic data calculation for 488 levels, their radiative transition probabilities and weighted oscillator strengths, autoionizing resonances belonging to two Rydberg series, and their associated parameters. The photoionization cross sections for all 488 fine-structure levels were obtained in length and velocity gauges. Results show that the 2p4d \({}^{1}\)P level is the lowest autoionizing resonance as obtained from quantum defect calculation. After a careful survey of the existing literature, we found that none of the published articles report fine-structure photo-absorption or photorecombination calculations for C III out of the 2s5s \({}^{1}\)S excited state. The present work shows that the contributions from associated bound-bound excitation and the bound-free processes on the photorecombination energy profile in the vicinity of 2p\(nd\)\({}^{1}\)P\({}_{0}\)\({}_{1}\) autoionizing resonances, as an example, are not comparable in magnitude, even for this simple system.