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

Vibrational Excitation Hindering an Ion-Molecule Reaction: The c-C_{3}H_{2}^{+}-H_{2} Collision Complex

Experiments within a cryogenic 22-pole ion trap have revealed an interesting reaction dynamic phenomenon, where rovibrational excitation of an ionic molecule slows down a reaction with a neutral partner. This is demonstrated for the low-temperature hydrogen abstraction reaction c-C_{3}H_{2}^{+}+H_{2}, where excitation of the ion into the ν_{7} antisymmetric C-H stretching mode decreased the reaction rate coefficient toward the products c-C_{3}H_{3}^{+}+H. Supported by high-level quantum-chemical calculations, this observation is explained by the reaction proceeding through a c-C_{3}H_{2}^{+}-H_{2} collision complex in the entrance channel, in which the hydrogen molecule is loosely bound to the hydrogen atom of the c-C_{3}H_{2}^{+} ion. This discovery enables high-resolution vibrational action spectroscopy for c-C_{3}H_{2}^{+} and other molecular ions with similar reaction pathways. Moreover, a detailed kinetic model relating the extent of the observed product depletion signal to the rate coefficients of inelastic collisions reveals that rotational relaxation of the vibrationally excited ions is significantly faster than the rovibrational relaxation, allowing for a large fraction of the ions to be vibrationally excited. This result provides fundamental insight into the mechanism for an important class of chemical reactions, and is capable of probing the inelastic collisional dynamics of molecular ions.

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

We present in this paper a detailed theoretical and computational analysis of the quantum inelastic dynamics involving the lower rotational levels of the MgH⁻ (X¹Σ⁺) molecular anion in collision with He atoms which provide the buffer gas in a cold trap. The interaction potential between the molecular partner and the He (¹S) gaseous atoms is obtained from accurate quantum chemical calculations at the post-Hartree-Fock level as described in this paper. The spatial features and the interaction strength of the present potential energy surface (PES) are analyzed in detail and in comparison with similar, earlier results involving the MgH⁺ (¹Σ) cation interacting with He atoms. The quantum, multichannel dynamics is then carried out using the newly obtained PES and the final inelastic rats constants, over the range of temperatures which are expected to be present in a cold ion trap experiment, are obtained to generate the multichannel kinetics of population changes observed for the molecular ion during the collisional cooling process. The rotational populations finally achieved at specific temperatures are linked to state-selective laser photo-detachment experiments to be carried out in our laboratory.All intermediate steps of the quantum modeling are also compared with the behavior of the corresponding MgH⁺ cation in the trap and the marked differences which exist between the collisional dynamics of the two systems are dicussed and explained. The feasibility of the present anion to be involved in state-selective photo-detachment experiments is fully analyzed and suggestions are made for the best performing conditions to be selected during trap experiments.

Modeling Quantum Kinetics in Ion Traps: State-changing Collisions for OH+ (3Σ- ) Ions with He as a Buffer Gas

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 10.1080/00268976.2018.1442597) 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.