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Journal of Physics B Atomic Molecular and Optical Physics

State-changing processes for ions in cold traps: LiH−molecules colliding with He as a buffer gas

Lola González‐Sánchez, Franco Antonio Gianturco, Roland Wester

Published Nov 4, 2016• 3 citations
Publisher

Abstract

We report in the present study a quantum analysis of the collisional dynamics involving a negative ion, LiH − in its 2 Σ + ground electronic state, and He as a buffer gas in the environment of cold ion traps. The work focuses on the evaluation of the internal cooling collisional rates, treating both the anion's rotational quantum numbers and the spin-changing processes. The calculations are carried out over a range of energies capable of yielding the corresponding rates for state-changing events over a rather broad interval of temperatures, thus covering those usually reached in the cold traps experiments and even beyond to lower temperatures.

DOI
10.1088/0953-4075/49/23/235201

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Faraday Discussions•Jan 1, 2018•13 citations
We report in the present study a quantum analysis of the collisional dynamics involving a negative ion, LiH − in its 2 Σ + ground electronic state, and He as a buffer gas in the environment of cold ion traps. The work focuses on the evaluation of the internal cooling collisional rates, treating both the anion's rotational quantum numbers and the spin-changing processes. The calculations are carried out over a range of energies capable of yielding the corresponding rates for state-changing events over a rather broad interval of temperatures, thus covering those usually reached in the cold traps experiments and even beyond to lower temperatures.
We present in this paper a detailed theoretical and computational analysis of the quantum 4 inelastic dynamics involving the lower rotational levels of the MgH-(X1Sigma+) molecular anion in 5 collision with He atoms which provides the buffer gas in a cold trap. The interaction potential 6 between the molecular partner and the He (1S) gaseous atoms is obtained from accurate 7 quantum chemical calculations at the post-Hartree-Fock level as described in this paper. The 8 spatial features and the interpretation strength of the present potential energy surface (PES) 9 are analysed in detail and in comparison with similar, earlier results involving the MgH+ (1Sigma) 10 cation interacting with He atoms. The quantum, multichannel dynamics is then carried out using 11 the newly obtained PES and the final inelastic rats constants, over the range of temperatures 12 which are expected to be present in a cold ion trap experiment, are obtained to generate the 13 multichannel kinetics of population changes observed for the molecular ion during the collisional 14 cooling process. The rotational populations finally achieved at specific temperatures are linked 15 to state-selective laser photo-detachment experiments to be carried out in our laboratory. All 16 intermediate steps of the quantum modelling are also compared with the behaviour of the 17 corresponding MgH+ cation in the trap and the marked differences which exist between the 18 collisional dynamics of the two systems are discussed and explained. The feasibility of the present 19 anion to be involved in state-selective photo-detachment experiments is fully analysed and 20 suggestions are made for the best performing conditions to be expected during trap experiments.
Using accurate ab initio calculations of the interaction forces, we employ a quantum mechanical description of the collisional state-changing processes that occur in a cold ion trap with He as a buffer gas. We generate the corresponding inelastic rates for rotational transitions involving three simple molecular anions OH−(1Σ), MgH−(1Σ), and C2H−(1Σ) colliding with the helium atoms of the trap. We show that the rotational constants of these molecular anions are such that within the low-temperature regimes of a cold ion trap (up to about 50 K), a different proportion of molecular states are significantly populated when loading helium as a buffer gas in the trap. By varying the trap operating conditions, population equilibrium at the relevant range of temperatures is reached within different time scales. In the modeling of the photodetachment experiments, we analyze the effects of varying the chosen values for photodetachment rates as well as the laser photon fluxes. Additionally, the changing of the collision dynamics under different buffer gas densities is examined and the best operating conditions, for the different anions, for yielding higher populations of specific rotational states within the ion traps are extracted. The present modeling thus illustrates possible preparation of the trap conditions for carrying out more efficiently state-selected experiments with the trapped anions.
Using the ab initio computed potential energy surface for the electronic interaction of the ${\mathrm{MgH}}^{+}(^{1}\mathrm{\ensuremath{\Sigma}}$) ion with the He(1S^{1}S1S) atom, we calculate the relevant state-changing rotationally inelastic collision cross sections from a quantum treatment of the multichannel scattering problem. We focus on the quantum dynamics at the translationally low energies for the present partners discussed in earlier, cold ion trap experiments (see below), which we wish to model in detail. The corresponding state-changing rates computed between the lower rotational states of the molecular ion are employed to describe the time-evolution kinetics followed by recent experiments on Coulomb-crystallized ${\mathrm{MgH}}^{+}(^{1}\mathrm{\ensuremath{\Sigma}}$), where the ions are rotationally cooled by micromotion tuning after being uploaded into the trap of He as a buffer gas. The present computational modeling of the final ion rotational temperatures in the experiments turns out to agree very well with the observations and points at a fast equilibration between rotational and thermal temperatures of the ions.
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‐( ) state, and then compares our findings with similar data for the OH−( ) 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.
We report close-coupling (CC) quantum dynamics calculations for collisional excitation/de-excitation of the lowest four rotational levels of OD − and of OH − interacting with Rb atoms in a cold ion trap. The calculations are carried out over a range of energies capable of yielding the corresponding rates for state-changing events over a rather broad interval of temperatures which cover those reached in earlier cold trap experiments. They involved sympathetic cooling of the molecular anion through a cloud of laser-cooled Rb atoms, an experiment which is currently being run again through a Heidelberg–Innsbruck collaboration. The significance of isotopic effects is analysed by comparing both systems and the range of temperatures examined in the calculations is extended up to 400 K, starting from a few mK. Both cross sections and rates are found to be markedly larger than in the case of OD − /OH − interacting the He atoms under the same conditions, and the isotopic effects are also seen to be rather significant at the energies examined in the present study. Such findings are discussed in the light of the observed trap losses of molecular anions.
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.