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Long-distance charge-dipole attraction between atomic ion and randomly oriented polar molecule potentially makes the molecular orientation, which profoundly influences the products' kinetics of collisional reaction. Using the three-dimensional ion velocity map imaging technique, here we report a collision-energy dependent stereodynamics of dissociative charge exchange reaction Ar + + CO → Ar + O + C + in a range of 7.46-9.97 eV. At the lowest collision energy, the most C + products are forward-scattered and are along the collision axis and are attributed to three different dissociation channels including the predominant one experiencing the rotating intermediate ArC + . At the high collision energies, the remarkably diffusive distribution of C + arises from the prompt dissociation of the rebounded CO + . The different dynamic processes arising from the nearly collinear collision are elaborated explicitly on the basis of the data analyses using the Doppler kinetics models.

Stereodynamics of the collisional reaction between mutually aligned or oriented reactants has been a striking topic of chemical dynamics for decades. However, the stereodynamic aspects are scarcely revealed for the low-energy collision with a randomly oriented target. Here in the dissociative charge-exchange reaction between randomly oriented O 2 and low-energy Ar + , we, using the three-dimensional ion velocity map imaging technique, clearly observe a linear alignment and a nearly isotropic distribution of the O + yields along the collision axis. These observations are rationalized with the Doppler kinetic models in which the O 2 bond is assumed to be parallel or unparallel to the collision axis of the large impact parameter collision. The linearly aligned O + , as the predominant yield, is produced in the parallel collision, while a rotating O 2 + , as the intermediate in the unparallel collision, leads to the isotropic distribution of O + .

Ion-molecule charge-exchange reactions Ar + + CO → Ar + CO + at the center-of-mass collision energies of 4.40, 6.40, and 8.39 eV are investigated using ion velocity map imaging technique. Although multiple electronically excited states of CO + are accessed, the population of CO + at the A 2 Π state is predominant in the present collision-energy range. In contrast to our previous study for NO, but similar to the case of O 2 , the forward-scattered CO + yields show a broader angular distribution at the higher collision energy. Typically, the Franck-Condon-region charge transfer, energy resonant charge transfer, and intimate collision are three different mechanisms in which the intimate collision experiences an intermediate complex, and this mechanism usually plays an essential role in the thermal-energy reactions. However, the present observations indicate that this mechanism, concerning the intermediate (Ar-CO) + , is still of utmost importance in a relatively high collision-energy range.

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.

High-lying quantum states of molecule are apt to be populated by translational-to-internal energy transfer in the collisions with atom, which usually becomes more significant with the increase of collision energy. However, in the charge exchange reaction Ar + + NO → Ar + NO + , the products NO + prefer a dominant population at the lowest triplet state a 3 Σ + , in particular, in the higher energy collisions; the higher states b 3 Π and w 3 Δ of NO + are accessed only at the lower collision energies. Such a striking collision-energy dependence is attributed to two distinctly different processes: the former is controlled with an energetically resonant charge-transfer mechanism; while the latter experiences an intermediate complex (Ar-NO) + which permits a more efficient translational-to-internal energy transfer.