Rotationally Inelastic Collisions of CN⁻ with He: Computing Cross Sections and Rates in the Interstellar Medium

Abstract

A newly calculated ab initio potential energy surface is used to compute collision-driven state-changing cross sections and rate coefficients over a range from 5 to 100 K for CN−(1Σ), the smallest anion detected in the interstellar medium, interacting with He, an abundant species in this environment. We compare our presently computed rate coefficients with those previously published for the similar and important systems CN–He, CN-H2, and CN−–H2 to illustrate the broader network of inelastic, state-changing processes for these four systems. We also discuss the size-scaling effects that occur when changing partners from He to H2. We further analyze the differences in size between collision-driven rate coefficients when going from neutral CN to its anion. All the present results are discussed in detail, to provide accurate and realistic data for chemical networks that wish to include the CN− anion in their modeling of astrochemical environments.

Collision-driven state-changing efficiency of different buffer gases in cold traps: He(1S), Ar(1S) and p-H2(1Σ) on trapped CN−(1Σ)

“…In this case, only the leading terms of the 4D multipolar expansion needs to be kept, thereby allowing the resulting RR-PES to be simplified to the lower 2D dimensionality as indicated by eqn (1). 16,29 We will not report this 2D PES again here, but note that its general features are stronger than in the case of He and more similar to those shown above for the Ar partner. Such differences will be further analysed in the following sections.…”

“…1 a pictorial representation of the new RR-PES compared with one describing the interaction with He atoms, already described in detail in our earlier work. 16 We will therefore not report again all aspects of those calculations but only show the results in comparison with the new PES for CN À /Ar. In both systems, we see that the largest attractive wells are located along the linear structures, with the deeper one on the side of the N-atom of the anion.…”

“…2 reports the lower six terms of expansion of eqn (1) for the newly computed interaction of Ar with CN À , while the panel on the right shows for comparison the same terms but for the CN À /He interaction computed previously. 16 The following comments can be made: (i) the relative depths of the two spherical terms of the multipoles is clearly visible in the inset on the left panel for Ar and in the right panel for the He partner. The one for the Ar atom is about one order of magnitude larger than that in the case of He; (ii) the next two anisotropic terms with l = 1 and 2 also show for the Ar case the presence of strongly attractive wells and steeply repulsive walls at short range, while the data for He in the right panel show instead that only the l = 2 term has a shallow well before the repulsive region; (iii) the next three anisotropic coefficients for the Ar PES remain uniformly repulsive at short range with increasingly shallower wells slightly further out.…”

“…29 For collisions at low temperatures (T r 100 K), the rotational excitation probability of H 2 itself is fairly low since the energy spacing between the j = 0 and j = 2 levels in p-H 2 is much larger than in the case of CN À . 16,29 We shall therefore only consider as a partner of the anion the p-H 2 ( j = 0) system. In this case, only the leading terms of the 4D multipolar expansion needs to be kept, thereby allowing the resulting RR-PES to be simplified to the lower 2D dimensionality as indicated by eqn (1).…”

“…Recent experimental work in our group has also been directed to photo-detachment studies of the CN À anion with He as the buffer gas of choice. 15,16 In the present study, we shall therefore revisit the behaviour of the CN À anion collisional inelastic rate constants involving rotational states active at the temperatures we shall consider for the cold traps. In particular, we shall obtain from first principle quantum calculations the relative sizes of its state-changing rotationally inelastic rate coefficients when using He, Ar or p-H 2 as buffer gases in the traps.…”