Reducing Si population in the ISM by charge exchange collisions with He+: a quantum modelling of the process

Abstract

The possible losses of silicon atom population during star-forming evolution and in the (photondominated region) PDR environments of the interstellar medium (ISM) can have different origins, one being the charge exchange (CE) encounter with the helium cations (Si + He + ) one of the most abundant species in those environments. This work investigates the different features of the likely interaction potentials leading to asymptotic partners like Si, Si + ,S i ++ and He or He + , in order to determine the influence of more accurate cross-sections on the chemical evolution of the ISM. We analyse the behaviour of interacting Si and He atoms by using ab initio quantum molecular methods. To obtain the corresponding transition probabilities, we employ a simple sequential grouping of single-crossing Landau–Zener events, and the timedependent rate coefficients for the CE processes involved are obtained over a broad range of the gas temperatures. The results are seen to differ substantially from an earlier Langevintype modelling of such process and further suggest a much more complex variety of possible molecular evolution mechanisms. We find, in fact, the unexpected presence of electron shakeoff effects leading to Si ++ generation yielding to emission of an electron, which has never been considered before and that, although with markedly smaller cross-sections, can indeed contribute to Si losses after the primary CE event. The consequences of these novel findings are tested on evolutionary model calculations and the results discussed in detail.