The effect of recurrent fluorescence on the survival probability of hot octasulfur cations

Context. Sulfur allotropes are proposed as a contributor to the missing atomic sulfur in the interstellar medium, in particular octasulfur, regarded as the most stable sulfur allotrope. Models of sulfur chemical pathways in the ISM strongly rely on the stability of octasulfur. Aims. We investigate the de-excitation dynamics of hot S8+ ions in the gas-phase, addressing experimentally the competition between fragmentation, vibrational cooling, and recurrent fluorescence. By combining the experiments with calculations of rate coefficients, we elucidate the survival probability of octasulfur, placing constraints on the interstellar environments where the allotrope can be detected. Methods. Experiments are performed at the cryogenic ion beam storage ring DESIREE at Stockholm University. Thermally hot S8+ ions are injected into the storage ring, where the time-dependent fragmentation yield is measured. In addition, using inputs from quantum chemical calculations, the energy-dependent rate coefficients of fragmentation, vibrational cooling, and recurrent fluorescence are modeled. The dynamics of the excited-state populations in S8+ is studied based on a master equation formalism, providing the survival probability of the ions as a function of internal energy. The analysis is also extended to the neutral S8 allotrope. Results. We observe that radiative cooling is efficient on a time scale of 95 ms, corresponding to internal energies in the range of 1.1-1.4 eV. Experimentally, two fragmentation channels are observed, following the pathways S8+ → S5+ + S3 and S8+ → S6+ + S2, in agreement with previous studies. From the computations, these channels have dissociation energies of 1.8 and 1.4 eV, respectively. The modeled rate coefficients predict very slow vibrational cooling, whereas those of recurrent fluorescence are consistent with the experiment. Recurrent fluorescence is expected to proceed from an electronic state at 0.76 eV, computed by time-dependent density functional theory. The analysis reveals a survival probability of less than 1% at internal energies higher than 1.70 eV. In contrast, calculations predict that recurrent fluorescence is not a competitive cooling channel in neutral S8, which promptly fragments above 1.4 eV. Conclusions. Recurrent fluorescence is predicted to be a competitive cooling channel for S8+, providing a stabilizing pathway for ions in hot environments. Based on simulated survival probabilities, an upper limit of 1100 K is found for the temperature of interstellar environments where S8+ could be detected. However, recurrent fluorescence is not effective for neutral S8, which will fragment already at internal energies close to its dissociation limit.