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

Abstract

Views of quantum potentials for CN− with He and Ar. Dynamics of the anion's rotational state-changes models cooling kinetics for either buffer gases in cold traps.

Vibrational quenching of CN− in collisions with He and Ar

“…25 Collisional processes of the anion with the astrochemically relevant He and H 2 species 33,34 for rotational transitions have recently been studied and we have also investigated the rotational cooling of this molecular anion with He, Ar, and H 2 as buffer gasses. 35 The CN − anion is also thought to be an important participant as well in reactions in the interstellar medium (ISM) [36][37][38][39] and in the atmosphere of Titan 40 where it has been detected. 41,42 We note in passing that the corresponding neutral species CN was one of the first molecules to be detected in space 43 and cross sections and rates for this species have been investigated and obtained for various rovibrational process in collisions with He and H 2 .…”

Beyond the helium buffer: 12C−2 rotational cooling in cold traps with H2 as a partner gas: interaction forces and quantum dynamics

“…Figure 11 shows a similar simulation but for a gas temperatures of 60 K. At this higher temperature, the thermalisation times are slightly shorter as the populations are closer to their initial values at 100 K. The results are similar to those at 20 K with H 2 providing the fastest thermalisation time of just under 1 second while for Ar, around 2 s are required. Again the low-pressure H 2 simulation shows thermalisation times which are somewhat comparable to those given when using He at the higher gas pressure indicated in the Figure . It is also of interest to compare the results presented here for the C − 2 thermalisation process in the traps to those obtained for the CN − anion, for which we have already carried out similar simulations in our earlier work [34]. The trends shown by those results turn out to be essentially the same, with the thermalisation times induced on the latter molecular anion by the p-H 2 (j = 0) partner gas being about twice as fast as those due to Ar and He, with He again turning out to be the slowest.…”

“…For example, the cross-sections and rate coefficients for rotationally inelastic transitions in the case of C − 2 colliding with Li and Rb were found to be quite similar to each other [26] and far larger (between 4 and 10 times) than those reported here for the p-H 2 (j=0) partner, as a consequence of the much stronger interactions shown by the former partners as we had mentioned earlier in this paper. The CN − anion can also be considered for comparison since we had found that its rotationally inelastic collisions with p-H 2 (j = 0) and with Ar yield cross-sections and rate coefficients which are more similar with each other than what we have found with C − 2 , particularly for the low N transitions [34].…”

“…Another possibility is to use hydrogen gas, H 2 , which has been used more often than argon and which is the focus of the present computational work. For the similar anion, CN − , we had found earlier that the rotationally inelastic rate coefficients for collisions with H 2 were even larger than those for argon, leading to fast thermalisation times [34], thereby suggesting H 2 may also be suitable as an efficient buffer gas for C − 2 . Hydrogen gas has the advantage of being usable down to about 10 K before freezing out.…”

“…In Figure 10, the vertical dashed line is used to indicate the time when the rotational states reach thermal equilibrium which was defined to be when the N = 0 state's population n 0 remains unchanged in the forth decimal place. This definition is the same as that chosen in our previous work on CN − /He/Ar/H 2 collisions [34] and allows comparisons between these systems.…”