摘要：Context. Helium, the second most abundant element in the Universe, with a relative abundance of He/H ~ 1/10, has never been observed in any other form than that of a neutral atom (He) or an ion (He+) in the interstellar medium. Since He is a noble gas its non-observation as part of neutral molecular systems is understandable, but it is very surprising for a positively charged species such as HeH+ that is a stable diatomic ion whose spectral signatures are well known in the laboratory.
Aims. This non-observation, even in hydrogen rich regions, could imply that HeH+ is not a proper target and that alternatives have to be considered, such as small HeH clusters. The present study aims at finding whether the leading term HeH fulfills the conditions required.
Methods. We addressed the question with state-of-the-art numerical simulations. We determined a two-dimension ab initio potential energy surface (PES) of the HeH cluster along the He...H and HeH+...H2 reaction coordinates. The calculations rely on complete active space configuration interaction followed by a second order perturbation treatment (CAS-PT2). This surface was used for the evaluation of the two radiative associations rate constants by means of a quantum treatment of the collision between the interacting fragments.
Results. These calculations show unambiguously that HeH is the most stable point on the corresponding global PES. Then, we determined the rate constants of the radiative associations HeH+ + H2 and He + H leading to HeH.
Conclusions. Significative values were obtained that reach up to 2 × 10-18 cm3 s-1, which should stimulate new tentatives to detect molecular helium in astrophysical objects.
