The role of intersystem crossing in the reactive collision of S+(4S) with H2

We report a study on the reactive collision of S+(4S) with H2, HD, and D2 combining guided ion beam experiments and quantum-mechanical calculations. It is found that the reactive cross sections reflect the existence of two different mechanisms, one being spin-forbidden. Using different models, we demonstrate that the spin-forbidden pathway follows a complex mechanism involving three electronic states instead of two as previously thought. The good agreement between theory and experiment validates the methodology employed and allows us to fully understand the reaction mechanism. This study also provides new fundamental insights into the intersystem crossing process.