HeH+ Collisions with H2: Rotationally Inelastic Cross Sections and Rate Coefficients from Quantum Dynamics at Interstellar Temperatures

We report for thefirst time an accurate ab initiopotential energy surface for the HeH+-H2system in fourdimensions (4D) treating both diatomic species as rigid rotors.The computed ab initio potential energy point values arefittedusing an artificial neural network method and used in quantumclose coupling calculations for different initial states of both rotors,in their ground electronic states, over a range of collision energies.The state-to-state cross section results are used to compute the ratecoefficients over a range of temperatures relevant to interstellarconditions. By comparing the four dimensional quantum resultswith those obtained by a reduced-dimensions approach that treatsthe H2molecule as an averaged, nonrotating target, it is shown thatthe reduced dimensionality results are in good accord with the four dimensional results as long as the HeH+molecule is not initiallyrotationally excited. By further comparing the present rate coefficients with those for HeH+-H and for HeH+-He, we demonstratethat H2molecules are the most effective collision partners in inducing rotational excitation in HeH+cation at interstellartemperatures. The rotationally inelastic rates involvingo-H2andp-H2excitations are also obtained and they turn out to be, as inprevious systems, orders of magnitude smaller than those involving the cation. The results for the H2molecular partner clearlyindicate its large energy-transfer efficiency to the HeH+system, thereby confirming its expected importance within the kineticsnetworks involving HeH+in interstellar environments