Near-resonant rotational energy transfer in HCl–H2 inelastic collisions

Abstract

We present a new four-dimensional (4D) potential energy surface for the HCl-H2 van der Waals system. Both molecules were treated as rigid rotors. Potential energy surface was obtained from electronic structure calculations using a coupled cluster with single, double, and perturbative triple excitations method. The four atoms were described using the augmented correlation-consistent quadruple zeta basis set and bond functions were placed at mid-distance between the HCl and H2 centers of mass for a better description of the van der Waals interaction. The global minimum is characterized by the well depth of 213.38 cm(-1) corresponding to the T-shape structure with H2 molecule on the H side of the HCl molecule. The dissociation energies D0 are 34.7 cm(-1) and 42.3 cm(-1) for the complex with para- and ortho-H2, respectively. These theoretical results obtained using our new PES are in good agreement with experimental values [D. T. Anderson, M. Schuder, and D. J. Nesbitt, Chem. Phys. 239, 253 (1998)]. Close coupling calculations of the inelastic integral rotational cross sections of HCl in collisions with para-H2 and ortho-H2 were performed at low and intermediate collisional energies. Significant differences exist between para- and ortho-H2 results. The strongest collision-induced rotational HCl transitions are the transitions with Δj = 1 for collisions with both para-H2 and ortho-H2. Rotational relaxation of HCl in collision with para-H2 in the rotationally excited states j = 2 is dominated by near-resonant energy transfer.

Collisional excitation of NH3 by atomic and molecular hydrogen

“…The accurate recoupling theory (see e.g. Faure & Lique (2012)) should be therefore extended to the case of moleculemolecule collisions, as in Lanza et al (2014) for the HCl-H2 system. The statistical method (in which the hyperfine selective rate coefficients are taken to be proportional to the statistical weights of the final states) can be also employed as a simple approximation.…”

“…Assuming the (reduced mass) scaling law is valid, the rate coefficients for p-H2(j2 = 0) and H should be about 50% and 80% larger than the He rate coefficients, respectively, owing to the smaller collisional reduced masses. However, as expected and found for many others hydrides like HCl (Lanza et al 2014), H2O (Daniel, Cernicharo & Dubernet 2006), etc. it can be seen in Fig.…”

Rotational excitation of HCN by para- and ortho-H2

“…This trend was already observed for the isomer HNC 29 and for several interstellar species like SiS, 40 CO, 41 or HCl. 42 The significant j 2 dependence of the cross sections is due to the large anisotropy of the PES with respect to H 2 rotations (see Ref. 30).…”

Perspective: Ultracold molecules and the dawn of cold controlled chemistry

“…Near-resonant processes involving pure rotational energy transfer have been observed in collisions of rotationally excited HCl( j 1 = 7) with para-H 2 ( j 2 = 2) leading to HCl( j 1 = 9) + H 2 ( j 2 = 0). 166 The energy deficit for the process is about 5.2 K and the process becomes dominant for energies below 100 K. Extensive calculations of rotational relaxation of CO by collisions with ortho-and para-H 2 have been reported by Yang et al 167 for CO rotational levels j = 1-40 within the rigid rotor formalism. The relaxation pathway was found to be mostly dominated by ∆ j = −1 transitions, for temperatures in the 1-1000 K range.…”