Hyperfine excitation of HCl by para-H2(j=0) (Lanza et al, 2014)

Display rate coefficients as : Display rate coefficients graphically : Data information :
  • HCl initial level labelled from 2 to 20
  • HCl final level labelled from 1 to 19
  • H2 initial level labelled from 1 to 1
  • H2 final level labelled from 1 to 1
  • 8 temperatures between 10 K and 300 K
  • Units : cm3 s-1





Reduced Mass = 1.9087 a.m.u.

References :


The calculation of the PES assumes that both HCl and H2 are rigid rotors. The molecular distances are fixed to their average values in the fundamental vibration mode. For H2, this distance is rHH = 1.45 a0 and for HCl, the distance is rHCl = 2.43 a0.

The HCl-H2 PES was calculated at the CCSD(T) / aug-cc-pVQZ level of theory. The basis was supplemented with mid bond functions. The basis set superposition error (BSSE) was corrected at all geometries with the Boys and Bernardi counterpoise procedure (Boys and Bernardi 1970).

The calculations were performed at 39 fixed distances in the range 3.5-50 a0. At each distance, the angular dependence was sampled on a predefined grid for the 3 angles that describe the 4D PES. A total of ~26400 ab initio points were computed. The analytical behaviour of the PES was obtained by fitting the ab intio points with the expression reported in Green et al. 1975, and, at every intermolecular distance R, consists in 174 angular terms vl1,l2,l(R). The errors reported for the fitting procedure are low: ~0.01% for the long range part of the PES, ~0.1% in the potential well and ~1% in the repulsive short range part of the PES. The radial coefficients vl1,l2,l(R) are subsequently fitted along the R coordinate, with polynomials of 1/Rn where n=[2;12].

References :

Close-coupling (CC) calculations are performed for total energies ET < 3000 cm-1, and solving the spin-free coupled set of differential equations. The parameters of the integrator (integration step and intermolecular distance boundaries) were adjusted to ensure a good convergence of the cross sections (the convergence criteria are however not specified).

Hyperfine cross sections are obtained through a recoupling technique, in a two step procedure. First, CC calculations are performed just taking into account the rotational energy levels of HCl, which leads to obtain the scatering matrix elements SJ(jl,j'l'). The hyperfine structure is subsequently introduced and the scattering matrix elements are recoupled according to the methodology described by Corey & McCourt 1983. This method is independent of the exact values taken by the hyperfine energy levels (rotational energies are assumed). If we assume that the difference in reduced mass between the H35Cl-H2 and H37Cl-H2 does not have a significant impact on the calculation of the SJ(jl,j'l') matrix elements, this implies that hyperfine de-excitation cross sections are identical for both H35Cl and H37Cl, since the two chlorine isotopes have the same nuclear spins.

References :


0.5 to 3000 cm-1

The number of HCl rotational states was adjusted to ensure convergence and include at least 5 closed rotational levels.
The j=0 and 2 para-H2 rotational states were included at all energies. The tests performed showed that including the j=4 state modifies the cross sections by less than 5%, for inelastic cross sections (with respect to HCl) within the j=0 state of para-H2.

For the current PES, the accuracy is better than 5% for inelastic rotational transitions within the j=0 state of para-H2.
The use of the recoupling technique to provide hyperfine rate coefficients introduces an additional error with respect to a full CC calculation. The accuracy of such a method is however expected to be high (of the order or better than 1%) given the small difference in energy between hyperfine levels (tens of 0.001 cm-1), compared to the energy involved in a collision.


Hyperfine de-excitation rate coefficients are provided among the first 6 rotational levels of HCl (up to j=5), for collisions with para-H2(j=0) and for temperatures in the range 10-300K. Either the 35Cl and 37Cl nuclei have nuclear spins I=3/2 that split every rotational levels in 4 hyperfine levels for j>1. Rate coefficients thus consider a total of 20 hyperfine levels and apply to H35Cl. The same set of rate coefficients can be used for the H37Cl isotopologue: an additional error is however introduced (presumably of a few percents) due to the difference in reduced mass between the H35Cl-H2 and H37Cl-H2 systems.


NB : the main reference is displayed in red
PDF Version

Collision history

Status Version Date
Collision added into the database 1 2016-03-16