The Role of Resonant Excitation in Lanthanide Collision Strengths for Non-LTE Kilonova Modeling

Accurate modeling of kilonova spectra, particularly during the late nebular phases (> 4 days post-merger) dominated by non-local thermodynamic equilibrium (non-LTE) processes [1, 2], demands detailed collisional atomic data for r process elements like lanthanides. While electron impact excitation (EIE) proceeds via direct scattering, the indirect pathway of resonant excitation (RE) represents another crucial, complex mechanism. RE proceeds via electron capture into intermediate autoionizing states followed by radiative decay to a bound excited level. Calculating RE accurately is computationally intensive [3, 4], and the requisite atomic data have been largely unavailable for complex lanthanide ions, often leading to the omission of its contribution in astrophysical models despite its potential to dominate total EIE rates [5].

Continuing our systematic effort to generate accurate atomic data for lanthanides, we present large-scale calculations focusing specifically on quantifying the impact of RE on EIE collision strengths for multiple singly and doubly ionized species. We employ the fully relativistic distorted wave (DW) method within the Flexible Atomic Code (FAC) [6]. These calculations utilize improved atomic structures derived from optimized potentials [7] and calibrated energy levels [8]. The RE channels were incorporated via the Independent Process, Isolated Resonance DW framework (IPIRDW) [5].

Our results provide extensive new EIE datasets highlighting the RE contribution, demonstrating that this resonant pathway provides a substantial, often dominant, component to the total collision strength for many astrophysically relevant transitions in lanthanides under nebular kilonova conditions [3, 4]. This work supplies vital atomic data focused on the resonant channel, needed to significantly enhance the fidelity of non-LTE radiative transfer simulations and improve interpretations of late time kilonova spectra.

[1] Pognan et al. MNRAS 510, 3806 (2022)
[2] Gillanders et al. MNRAS 529, 2918–2945 (2024)
[3] M McCann et al. MNRAS 538, 537–552 (2025)
[4] Leo P. Mulholland et al. JQSRT 345, 109545 (2025)
[5] Li et al. Chin. Phys. B. 24, 113401 (2015).
[6] Gu et al. Can. J. Phys. 86, 675 (2008)
[7] Ferreira da Silva et al., Phys. Rev. A 112(1), 012802 (2025)
[8] A. Flörs et al., arXiv:2507.07785 (2025)