Investigation of critical parameters controlling the efficiency of associative ionization

Abstract

This paper compiles our merged-beam experimental findings for the associative ionization (AI) process from charged reactants, with the aim of guiding future investigations with e.g. the double electrostatic ion storage ring DESIREE in Stockholm. A reinvestigation of the isotopic effect in H−(D−) + He+ collisions is presented, along with a review of and NO+ production by AI involving ion pairs or excited neutrals, and put in perspective with the mutual neutralization and radiative association reactions. Critical parameters are identified and evaluated for their systematic role in controlling the magnitude of the cross section: isotopic substitution, exothermicity, electronic state density, and spin statistics.

Mutual Neutralization in Li+−D− Collisions: A Combined Experimental and Theoretical Study

“…The cross section agrees well with the previ-ous measurements by Peart & Hayton (1994) in the overlapping collision energy range (about 1 eV). Two regimes can be distinguished: below collision energies of 0.5 eV, the cross section behaves as E −1 as expected from the Wigner threshold law (Wigner 1948;Le Padellec et al 2017) and in agreement with previous mutual neutralization studies (Stenrup et al 2009;Nkambule et al 2016;Hedberg et al 2014), while above 0.5 eV the cross section becomes flatter. This behavior is well reproduced by our calculations based on the multichannel Landau-Zener approach with the molecular data obtained with the ACV5Z+G basis set, shown by the full line in Fig.…”

Reactions of C+ + Cl−, Br−, and I−—A comparison of theory and experiment

“…Associative ionization cross sections measured using merged beam techniques have been reported for N + and O + + Oand for various isotopomers of H + + Hand He + + H -. 16,18,19,60 In all cases, the energy dependence of the associative ionization cross sections is stronger than that for the 13 competing MN pathway such that the contribution of associative ionization is largest at low collision energies. For the systems involving only light atoms, the associative ionization cross section is <1% of the MN cross section at thermal energies, while for the heavy atom systems N + + Oand O + + Oassociative ionization cross sections are somewhat larger, but still only 1-2% of MN cross sections at thermal energies.…”

“…An estimate of maximum cross sections for MN and associative ionization has been derived by relating a maximum impact parameter to the internuclear distance at which the reactant Coulomb potential crosses the relevant product channel asymptote, with the resulting upper limits in good agreement with experimental results. 19 For the C + + Clsystem, where associative ionization is exothermic by ~2.2 eV, the estimated maximum rate constant at 300 K is 3.5 ×10 -9 cm 3 s -1 . While this is the magnitude of rate constant required to reconcile the experiment and theory, it is unlikely that associative ionization occurs at this limit.…”

“…The possibility may be evaluated in two ways: one, by theoretical treatment of the associative ionization pathway or, two, experimental study of the C " + Cl & reaction at thermal energies in a merged-beam apparatus with more direct access to product formation. 18,19…”

“…thermal energies, initially only for the H + + Hsystem, 16,17 but more recently for a wider range of atom-atom systems reporting both crosssections and product information. 18,19 Separately, an updated flowing afterglow methodology, developed by our group, termed variable electron and neutral density attachment mass spectrometry (VENDAMS) 20 provided an order of magnitude lower detection limit than the traditional flowing afterglow technique, allowed access to a broader range of systems, 21,22 as well as limited product information. 23 Recent efforts using VENDAMS have focused on atom-atom MN systems.…”