R-matrix benchmark study of SO2 photoionization dynamics

Abstract

Sulfur dioxide is a molecule of broad atmospheric and astronomical relevance, playing a central role in greenhouse warming and ozone chemistry. It is widely detected in planetary and interstellar environments and further participates in sulfur plasma and shock-driven processes. For this purpose, the ab initio R-matrix method within the close-coupling approximation is employed to investigate the photoionization dynamics of sulfur dioxide. Total and state-resolved cross sections for the three lowest ionic states, arising from ionization of the valence orbitals, are calculated, revealing rich autoionizing resonances in the near-threshold region. The high-resolution computed cross sections are benchmarked against available experimental datasets, including both direct measurements and reconstructed data, with the reported partial-channel cross sections representing the first direct state-resolved theoretical results for the dominant photoionization channels. In the absence of fully state-resolved experimental measurements for the cationic states of SO2, the results are interpreted through comparison with fragment-resolved data, enabling a direct correspondence between the computed ionic states and dominant molecular ion production channels, where the lowest three ionic states, X2A1(8a1−1), A2B2(5b2−1), and B2A2(1a2−1), contribute solely to the formation of the parent molecular ion SO2+. To assess the reliability of the results, a series of systematic benchmarks with respect to active space size, basis set, target-state expansion, and R-matrix radius confirms the convergence and robustness of the computed cross sections. The present results provide reliable reference data for modeling photochemical and radiative processes in planetary atmospheres and the interstellar medium and establish SO2 as a benchmark system for molecular photoionization studies.