Abstract
We present quantum scattering calculations for rotational state‐changing cross sections and rates, up to about 50 K of ion translational temperatures, for the OH+ molecular ion in collision with He atoms as the buffer gas in the trap. The results are obtained both by using the correct spin‐rotation coupling of angular momenta and also within a recoupling scheme that treats the molecular target as a pseudo‐(1Σ+ ) state, and then compares our findings with similar data for the OH−(1Σ+ ) molecular partner under the same conditions. This comparison intends to link the cation behaviour to the one found earlier for the molecular anion. The full calculations including the spin‐rotation angular momenta coupling effects have been recently reported (L. González‐Sánchez and R. Wester and F.A. Gianturco, Mol.Phys.2018, DOI 10.1080/00268976.2018.14425971) with the aim of extracting specific propensity rules controlling the size of the cross sections. The present study is instead directed to modelling trap cooling dynamics by further obtaining the solutions of the corresponding kinetics equations under different trap schemes so that, using the presently computed rates can allow us to indicate specific optimal conditions for the experimental setup of the collisional rotational cooling in an ion trap for the system under study.
Highlights
A central issue with the technique of buffer gas cooling is, the varying efficiency of cooling different internal degrees of freedom of the molecular target species (e. g.rotations, vibrations, spin-flips, etc) since the different statistics obviously can have drastic effects on the operational populations of the molecules in a desired state
( S ) in ultracold ion traps involving He as a buffer gas,[14] we had analyzed the interaction potential of these species by carrying out an extensive ab initio analysis and further illustrated the possible existence of propensity rules among the state-changing collisional cross sections involving either rotational or spin-flipping transitions within the partners
It is clear from the data reported in that Figure how the recoupling scheme produces rotationally inelastic cross sections with the same energy profiles as those obtained by using the full spin-rotational angular momentum coupling dynamics
Summary
A central issue with the technique of buffer gas cooling is, the varying efficiency of cooling different internal degrees of freedom of the molecular target species In a recent study carried out in our laboratory, for example, specific rotational state-changing collisions at low temperatures have been analysed forhydroxyl anions with He as a buffer gas during photodetachment experiments that manipulated molecular quantum states by non-resonant processes.[7] Rather good agreement was found there with the quantum modeling of the relevant dynamics and with our quantitative estimate of the involved collisional rates. ( S ) in ultracold ion traps involving He as a buffer gas,[14] we had analyzed the interaction potential of these species by carrying out an extensive ab initio analysis and further illustrated the possible existence of propensity rules among the state-changing collisional cross sections involving either rotational or spin-flipping transitions within the partners. Our present conclusions are given in our Section 7
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