NH2- in a cold ion trap with He buffer gas: Ab initio quantum modeling of the interaction potential and of state-changing multichannel dynamics

Modeling Ionic Reactions at Interstellar Temperatures: The Case of NH2- + H2 ⇔ NH3 + H. J Phys Chem A 2019;123:9905-9918. [PMID: 31633351 DOI: 10.1021/acs.jpca.9b07317]

Structural features and enthalpy details are presented for the title reactions, both for the exothermic (forward) path to NH₃ formation and for the endothermic (reverse) reaction to NH₂^- formation. Both pathways have relevance for the nitrogen chemistry in the interstellar medium (ISM). They are also helpful to document the possible role of H^- in molecular clouds at temperatures well below room temperature. The structural calculations are carried out using different ab initio methods and are further employed to obtain the reaction rates down to the interstellar temperatures detected in earlier experiments. The reaction rates are obtained from the computed minimum energy path (MEP) using the variational transition-state theory (VTST) approach. The results indicate very good accord with experiment results at room temperature, while measured low temperature data down to 8 K are well described using an appropriately modified VTST approach. This is done by employing a temperature-dependent scaling, from room temperature conditions down to the lower ISM temperatures, which acknowledges the noncanonical behavior of the fast, barrierless exothermic reaction. The reasons for this behavior and the need for improving on the VTST method when used away from room temperatures are discussed.

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.

Collisional relaxation kinetics for ortho and para NH2- under photodetachment in cold ion traps

The collisional cooling of the internal rotational states of the nonlinear anion NH2- (1A1), occurring at the low temperature of a cold ion trap under helium buffer gas cooling, is examined via quantum dynamics calculations and ion decay rate measurements. The calculations employ a novel ab initio potential energy surface that describes the interaction anisotropy and range of action between the molecular anions and the neutral He atoms. The state changing integral cross sections are employed to obtain the state-to-state rate coefficients, separately for the ortho- and the para-NH2- ions. These rates are in turn used to compute the state population evolution in the trap for both species, once photodetachment by a laser is initiated in the trap. The present work shows results for the combined losses of both species after the photodetachment laser is switched on and analyzes the differences of loss kinetics between the two hyperfine isomers.