• phys/phys.phys/phys.phys.phys-atm-ph
  
• phys/phys.phys/phys.phys.phys-chem-ph
  
• phys/phys.phys/phys.phys.phys-comp-ph
  

While the growing realization of the importance of long-range interactions is being demonstrated in cold and ultracold bimolecular collision experiments, their influence on one of the most critical ion-neutral reactions has been overlooked. Here, we address the non-Langevin abrupt decrease observed earlier in the low energy integral cross sections and rate coefficients of the astrochemically important H + HeH$^+$ $\rightarrow$ H$^+_2$ + He reaction. We attribute this to the presence of artificial barriers on existing potential energy surfaces (PESs). By incorporating precise long-range interaction terms, we introduce a new refined barrierless PES for the electronic ground state of HeH$_{2}^{+}$ reactive system, aligning closely with high level {\ab} electronic energies. Our findings, supported by various classical, quantum, and statistical methods, underscore the significance of long-range terms in accurately modeling reactive PESs. The low temperature rate coefficient on this new PES shows a substantial enhancement as compared to the previous results and aligns with Langevin behavior. This enhancement could affect noticeably the impact HeH$^{+}$ abundance prediction in early Universe condition.

Growing innovations in recent experimental studies of cold and ultracold gas-phase bimolec- ular collisions have demonstrated the importance of long-range interactions in ion-neutral reactions [1]. In this study, we address the non-Langevin abrupt decrease observed ear- lier [2,3] in the low energy integral cross sections and rate coefficients of astrochemically important H + HeH+ ! H+ 2 + He reaction. We attribute this to the presence of artificial barriers on existing potential energy surfaces (PESs) [4]. By including precise long-range interaction terms, we introduce a new refined barrierless PES for the HeH+ 2 reactive system, aligning closely with high level ab initio electronic energies. Our findings, supported by various classical, quantum, and statistical methods, underscore the significance of long- range terms in accurately modeling reactive PESs. Moreover, the substantial enhancement in rate coefficients at low temperatures (roughly 2 orders of magnitude at 10 K), which conform with Langevin behavior, may have significant impact on the HeH+ abundance prediction in early Universe condition, which are further to be discovered. Most notably, this enhancement yields a reaction rate coefficient of comparable magnitude to that observed in a recent experiment [5] on dissociative recombination (DR) of HeH+, particularly within 100 K. This suggests that the destruction of primordial HeH+ by H may become as significant as the DR process at low redshifts.