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

Cold and ultra-cold molecular systems are attracting the attention of researchers because the quantum nature of matter manifests itself clearly under these conditions, and research into such systems provides new insight into the quantum theory of matter and matter-light interaction. Experimental techniques for cooling, trapping and colliding cold and ultra-cold molecules continue to develop at an astounding pace. The applications of those investigations include for example, observing explicit quantum effects in chemical reactivity, developing an understanding of chemistry in cold astrophysical media and the use of very high-precision measurement to answer fundamental physics questions. Thus, an accurate theoretical treatment of cold chemical reactions is needed for the understanding of the reaction mechanisms and interpreting the experimental measurements. In the case of ion-neutral chemical reactions, a proper description of long range interactions is crucial for an accurate estimation of the reactive scattering process. The impact of accurate ab initio potentials will be highlighted by a rigorous study using various classical, quantum and statistical numerical method on the chemical reaction H + HeH$^{+}\rightarrow$ H$^{+}_2$ + He of interest in the context of early-universe chemistry.

The chemical reactions occurring in the astrophysical context are at the origin of the molecular complexity observed in these environments[1]. These chemical reactions are characterised by their thermal rate constants, which can be theoretically determined with quantum dynamics simulations. State-of-the-art, in quantum dynamics methods, remains the Close-Coupling approach[2-3] but is mainly limited to triatomic species (due to their computational cost) and requires a global potential energy surface. Although very accurate, this method is not suitable for more complex molecular systems and new computational methods, approximated but accurate, are needed. We propose a new set of curvilinear coordinates transformations[4] that enable us to perform quantum dynamics simulations following a minimum energy path. This approach do not require a full global potential energy surface calculation. This set of curvilinear coordinates are combined with a direct determination of the thermal rate constant using the Green's operator formulation with complex absorbing potentials[5] avoiding the full S-matrix computation. The efficiency of this new scheme is demonstrated on the collinear chemical reaction H + H2 → H2 + H for which accurate close-coupling results are available [6,7]. Furthermore, our computational scheme can be efficiently combined with a reduced dimensional model for complex chemical reactions.

we present an overview of two-layer anharmonicity in water dimer and in hydrogen molecule confined in clathrates

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.