Ion Momentum Imaging Study of the Ion–Molecule Charge Exchange Reaction of Ar⁺ + CO₂

Abstract

Three-dimensional ion momentum imaging is developed in a combination of ion velocity map imaging technique and delay-line anode ion detection, and it is applied for the ion−molecule charge exchange reaction between Ar + and CO 2 . In a center-of-mass collision energy range of 7.23−15.96 eV, CO 2 + products are primarily populated at the ground state X 2 Π g and the single-electron excited states A 2 Π u , B 2 Σ u + , and C 2 Σ g + ; the multielectron excited states of CO 2 + are also found at the higher collision energies. The production efficiency profiles of CO 2 + are distinctly different from the photoionization electron spectrum of CO 2 , implying that the charge transfer from Ar + would be not fast as expected. The strong electron correlations in the short-lived intermediate (Ar−CO 2 ) + should be responsible for the CO 2 + yields at the multielectron excited states.

Charge transfer dynamics in Ar+ + CO

“…They represent a simple reaction that occurs through the exchange of an electron, but can exhibit quite intricate mechanisms [1,2]. This becomes evident in a multitude of CONTACT R. Wester roland.wester@uibk.ac.at previously studied charge transfer reactions [3][4][5][6][7][8][9][10][11][12][13]. One particularly interesting one is the reaction of Ar + ( 2 P J ) with the diatomic CO molecule; a reaction that has been studied for almost six decades .…”

“…We have obtained differential cross sections for the title reaction by using our ion-molecule crossed beam velocity map imaging setup at the two collision energies 0.55 and 0.74 eV, bridging the gap between earlier studies at thermal and the recent study at higher collision energies. This experimental method has proven to be capable of disentangling the internal product energy distribution in numerous charge transfer reactions [3][4][5][6][7][8][9][10][11][12][13].…”

Stereodynamics Observed in the Reactive Collisions of Low-Energy Ar+ with Randomly Oriented O2

“…However, the stereodynamic aspects, i.e., vector correlations between the reactants and products, recede seriously if a slowly proceeding reaction involves an intermediate complex having a lifetime comparable to or longer than the molecular rotation periods, or they are scarcely visible in the collision of randomly oriented or rotational state-unaligned reactants . Here we report a crossed-beam experimental study of the dissociative charge-exchange (DCE) reaction between randomly oriented O 2 and low-energy ion Ar + (Ar + + O 2 → Ar + O + O + ) in which two different stereodynamic features are disentangled by the O + velocity imaging measurements with our three-dimensional ion velocity map imaging (3D-VMI) apparatus …”

“…Although the 3D-VMI technique has been successfully utilized for the reaction experiments, recently it was employed in our ion–molecule reaction apparatus . Our 3D-VMI experiments were carried out at center-of-mass (c.m.)…”

“…Such striking anisotropic distributions along the collision axis of the O + product are clearly observed for the first time in the Ar + + O 2 DCE reaction, owing to the application of the 3D-VMI technique. The prompt positive charge-exchange process approximates a virtual photon ionization of the target. , White circles in Figure mimic the isotropic momentum distributions of the O + ions produced in the dissociative photoionization of the free motionless O 2 at the image center. In the DCE reaction, some O + signals out of the white circle arise from the Doppler effect, i.e., the dissociation of the forward-moving O 2 + .…”