In the present study, the radiative association mechanism of interstellar silicon sulfide (SiS) was systematically investigated via high-level quantum chemical calculations. Specifically, the icMRCI+Q method combined with the aug-cc-pwCVTZ-DK basis set was employed to compute the potential energy curves (PECs) for 18 Λ-S states of SiS correlated to the Si(3Pg)+S(3Pg) dissociation limit. From these PECs, the spectroscopic constants of the SiS molecule were derived, and the calculated results showed excellent agreement with available experimental data and referenced theoretical values, thereby validating the reliability of the adopted computational approach.
Furthermore, the electric dipole transition moments (EDTMs) between the ground state (X¹Σ⁺) and three low-lying excited states(1¹Π, 2¹Π, 2¹Σ⁺) were characterized, and they were observed to exhibit a decay trend with increasing internuclear distance—a characteristic that is consistent with the neutral dissociation pathway of SiS, providing key insights into the molecular dissociation dynamics.
A core innovation of this work lies in the comprehensive quantum mechanical characterization of the resonance structures in the radiative association cross-sections of SiS. The resonance characteristics were explicitly correlated with the depth of the potential wells of the electronic states, and the microscopic formation mechanism of these resonance structures was fully elucidated from the quantum level. Specifically, deep potential well states (1¹Π, 2¹Σ⁺) were found to induce resonance envelopes in the energy range of 0.244-1.000 eV, whereas the shallow potential well state (2¹Π) generated sharp resonance peaks around 10^-3 eV.
To facilitate its application in astrochemical modeling, the radiative association rate coefficients of SiS were calculated over a broad temperature range (10-20000 K) and fitted to a three-term Arrhenius-Kooij parameterized formula, which can be directly embedded into mainstream astrochemical reaction networks. Collectively, this study fills the long-standing data deficiency of this key interstellar reaction, provides a comprehensive quantum mechanical understanding of the SiS radiative association process, and offers crucial thermochemical and dynamical data that are indispensable for simulating silicon-sulfur chemical processes in interstellar clouds and circumstellar environments.