Rotational ‘cooling’ and ‘heating’ of OH⁺(³Σ⁻) by collisions with He: quantum dynamics revealing propensity rules under ion trap conditions

Abstract

Multichannel scattering calculations are presented for the low-energy collisions of the OH+ cation and He atoms, using an ab initio evaluation of the interaction potential, which had been obtained in earlier work, and a time-independent, multichannel treatment of the quantum dynamics carried out in this study using our in-house scattering code ASPIN. Given the presence of spin-rotation coupling effects, within an essentially electrostatic formulation of the interaction forces with He atoms in the trap, the ensuing propensity rules which control the relative size of the state-changing cross sections and of the corresponding inelastic rates, also computed at the most likely temperatures in an ion trap, are presented and analysed in detail.

Collisional Quantum Dynamics for MgH− (1Σ+) With He as a Buffer Gas: Ionic State-Changing Reactions in Cold Traps

“…Very similar types of behavior could be seen for the state-changing rotation-cooling (de-excitation) rates shown by the data in the two panels of Figure 11: the Δ j =-1 transitions produce by far the largest rates over the selected range of temperatures. On the other hand, one also sees there that all rates are in the range of 10 −10 cm 3 s −1 , i.e., similar in size to those found earlier for the MgH + cation in cold traps under similar conditions (González-Sánchez et al, 2018b).…”

“…At least for the case of the three lowest rotational levels of the target molecule, it is also interesting to make a comparison between the cross sections behavior in the case of MgH − in the trap (present results) and those obtained for the case where the cation is instead in the trap (discussed by us in more detail in González-Sánchez et al, 2018b). The comparison is reported by the three panels of Figure 9 below.…”

“…By a numerical quadrature of the computed cross sections of equation 6, as indicate by equation 7, we can obtain the corresponding rates as a function of trap's temperature (González-Sánchez et al, 2015, 2018b; Schiller et al, 2017).…”

Modeling Quantum Kinetics in Ion Traps: State‐changing Collisions for OH+() Ions with He as a Buffer Gas

“…3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 One clearly sees from the relative energy spacings shown by Figure 2 that the DN6 ¼0 transitions involve much larger energy gaps than those transitions were only the S quantum number changes: spin-flip processes therefore involve much smaller energy transfers during inelastic collisions, as we shall further discuss below and as we have already illustrated in our earlier work. [1] We also see in that same Figure 2 that, by removing the spin-rotational coupling which causes the energy splitting terms, we could obtain an approximate picture for the rotational structure of the target molecule that becomes essentially described as a pseudo-1 S case, with the corresponding simplification of the energy spacings reported by the schematic energy ladder in the center of Figure 2. The validity of such a recoupling scheme was recently tested by us for the Hydrogen Molecular Ions (HMIs) in collision with He [19] and found to provide a very realistic description of the inelastic dynamics.…”

“…A further topic of interest in studying the inelastic collisional dynamics of the present system is provided by searching for possible propensity rules that would allow us to calibrate the relative sizes of the state-changing cross sections in the presence of the full couplings of the electronic spin and the rotational angular momenta, within the full angular momentum coupling dynamics. We have recently carried out such a study for this very system and have been able to computationally show the dominance of the pure spin-changing collisional probabilities [1] in relation to the other rotationally inelastic channels. In the present analysis, however, we shall extend the use of the quantum dynamics to further discuss the role of its computed rates within the master eq.s that analyse the temporal evolutions of the rotational state populations under a variety of conditions chosen to describe the ion trap and of which those discussed above have been an example.…”