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Physical Review A

Cooling and quenching of Mg24 H + ( X 1Σ+ ) by He4 ( 1S ) in a Coulomb trap: A quantum study of the dynamics

M. Tacconi, Franco Antonio Gianturco, Ersin Yurtsever, D. Caruso

Published Jul 13, 2011• 7 citations
Publisher

Abstract

The quantum dynamics of the rotational quenching of the $^{24}\mathrm{Mg}$H${}^{+}(X {}^{1}{\ensuremath{\Sigma}}^{+})$ molecular cation interacting with $^{4}\mathrm{He}$(${}^{1}S$) as a buffer gas, at relative temperatures ranging from 1 K down to millikelvins, is described by accurate close coupling scattering calculations on an ab initio potential energy surface. The efficiency of the process is quantitatively analyzed by employing the ab initio quenching rates and cross sections and effective decay rate values are obtained from simple unimolecular kinetics over a broad range of trap conditions.

DOI
10.1103/physreva.84.013412

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The quantum dynamics of the rotational quenching of the 24Mg^{24}\mathrm{Mg}24MgH+(X1\ensuremathΣ+){}^{+}(X {}^{1}{\ensuremath{\Sigma}}^{+})+(X1\ensuremathΣ+) molecular cation interacting with 4He^{4}\mathrm{He}4He(1S{}^{1}S1S) as a buffer gas, at relative temperatures ranging from 1 K down to millikelvins, is described by accurate close coupling scattering calculations on an ab initio potential energy surface. The efficiency of the process is quantitatively analyzed by employing the ab initio quenching rates and cross sections and effective decay rate values are obtained from simple unimolecular kinetics over a broad range of trap conditions.
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.
Following the previous study with an extensive range of quantum calculations involving different electronic states of the BN- anion [Dulitz et al., Phys. Scripta 100, 055411 (2025)], we now extend that work by modeling the quantum dynamics of the collision cooling of its rotational states in order to investigate possible paths for bringing this molecular anion down to temperatures of a few Kelvins. This specific ionic system is of direct interest when modeling experiments in cold ion traps where He or Ar atoms can function as the chief buffer gases that drive the anions down to the low trap temperatures. We employ accurate, ab initio calculations of the potential energy surfaces for the title system in its ground electronic state, interacting with either He or Ar atoms. We then obtain a wide range of inelastic cross sections and the ensuing rate coefficients in order to model the quantum kinetics of the time evolution of the cooling steps under different temperature and trap conditions. The results are analyzed and employed to estimate the cooling efficiency paths provided by various trap arrangements for the title anion. The results show that-using either of the two investigated species-the buffer gas cooling process very efficiently brings the anions to their lowest rotational states. These findings are very promising for future applications in the field of anion laser cooling.
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.
Quantum-scattering calculations at ultralow (close to ${10}^{\ensuremath{-}6}$ cm${}^{\ensuremath{-}1}$) collision energies are carried out for the Cs dimer in its spin-stretched triplet state, interacting with helium. An ab initio potential energy surface is computed and employed, while the target molecule is kept in its ground vibrational state and several excitated initial rotational states are considered in the quantum dynamics. The highly anisotropic interaction is seen to cause, in spite of its weakness, internal energy quenching rates comparable with the efficiency of the collisional cooling of relative kinetic energies. The rates of spin-flip processes are also analyzed and compared with pure rotational quenching events.
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.
Precision vibrational spectroscopy of the molecular hydrogen ions is of significant interest for determining fundamental constants, for searching for new forces, and for testing quantum electrodynamics calculations. Future experiments can profit from the ability of preparing molecular hydrogen ions at ultralow kinetic energy and in preselected internal states, with respect to vibration, rotation, and spin degrees of freedom. For the homonuclear ions (H2+,D2+{\mathrm{H}}_{2}{}^{+}, {\mathrm{D}}_{2}{}^{+}H2​+,D2​+), direct laser cooling of the rotational degree of freedom is not feasible. We show by quantum calculations that rotational cooling by cold He buffer gas is an effective approach. For this purpose we have computed the energy-dependent cross sections for rotationally elastic and inelastic collisions, ${\mathrm{h}}_{2}{}^{+}(v=0,\phantom{\rule{0.16em}{0ex}}N)+\mathrm{He}\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow} {\mathrm{h}}_{2}{}^{+}(v=0,\phantom{\rule{0.16em}{0ex}}{N}^{\ensuremath{'}})+\mathrm{He}$ (where h=H,D)\mathrm{h}=\mathrm{H},\phantom{\rule{0.16em}{0ex}}\mathrm{D})h=H,D), using ab initio coupled-channel calculations. We find that rotational cooling to the lowest rotational state is possible within tens of seconds under experimentally realistic conditions. We furthermore describe possible protocols for the preparation of a single quantum state, where also the spin state is well defined.