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The Journal of Chemical Physics

Modeling state-selective photodetachment in cold ion traps: Rotational state “crowding” in small anions

Franco Antonio Gianturco, Lola González‐Sánchez, Barry P. Mant, Roland Wester

Published Oct 10, 2019• 20 citations
Publisher

Abstract

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.

DOI
10.1063/1.5123218

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arXiv:2010.14458•Physical Review A•Nov 15, 2018•5 citations
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.
Abstract We present a computational investigation of the photodetachment (PD) dynamics of C2H− in its ground, electronic 1Σ state, cooled in an ion trap with He as a buffer gas. Our analysis employs a range of PD rates from quantum dynamics, selective choices of the anion’s initial rotational state populations as the laser is switched on, and different operational trap conditions. Our results show that a variety of outcomes for the initial state-dependent PD process is possible by selecting different initial conditions in the cold trap, the latter generating a dynamical interplay between the anion’s populations of rotational states, induced by the buffer gas, and the chosen laser frequency and laser power. Specific parameter selections during the PD process will be shown to cause different abundances of the rotational states of the anion in the trap. Graphical abstract
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.
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.
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 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.
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.