Ce II-IV emission in kilonovae with R-matrix collision strengths and distorted wave recombination rates
We provide transition probabilities and Maxwellian-averaged collision-strengths for the electron-impact excitation of Ce {\sc ii} - {\sc iv}. These ions are potentially abundant in kilonovae (KNe) and feature both forbidden and allowed transitions in the infrared. These atomic data are obtained using the {\sc grasp}$^0$ atomic structure code, interfaced with the non-perturbative {\sc darc} R-matrix close-coupling codes. We additionally use {\sc autostructure} to produce dielectronic and radiative recombination rate coefficients for Ce {\sc ii} - {\sc vi}. Using this data, we calculate an approximate ionization balance including non-thermal ionization rates with deposition rates from realistic models. We find Ce {\sc ii} - {\sc iv} to be of roughly equal abundance, subject to ejecta conditions. We find that generally we need $\gtrsim$ 10$^{-3}$ M$_{\odot}$ of Ce in total to produce emission of comparable luminosity to the features in the spectrum of AT2023vfi. In particular, it is found that [Ce {\sc iv}] 4.5 $μ$m requires both strong ionization and weak recombination, as well as a production of Ce considerably stronger than that of the solar abundance in order to have luminosity comparable to the apparent emission in this wavelength vicinity in \grb. %which is seemingly incompatible with the current understanding of the bulk composition of KNe. With such strict requirements on individual element masses, the detection of individual lanthanides on a line-by-line basis in the nebular phase seems unlikely - but lanthanides may remain important for cooling and early time analyses.