Optical excitation in collision of 500–5000 eV H2+, He+, Ne+, Ar+, and N2+ions with CO2

Ion Velocity Map Imaging Study of the Reactive Collisions between Carbon Dioxide and Helium Ion

The reactive collision between He+ and CO2 plays an important role in substance evolutions of the planetary CO2-rich atmosphere. Using a three-dimensional ion velocity map imaging technique, we investigate the low-energy ion-molecule reactions He+ + CO2 → He + CO2+/He + CO+ + O/He + CO + O+. The velocity images of the products CO+ and O+ of dissociative charge-exchange reactions are distinctly different from those of charge-exchange product CO2+. The remarkable features of stereodynamics are observed in the dissociative charge-exchange reaction and are attributed to the spatial alignment of the initially random target CO2 during the He+ approach. Branching ratios of different channels of dissociative charge exchange are further obtained with the Doppler kinematics model, indicating a high preference for the energy-resonant channel.

Should a Franck-Condon or a curve-crossing picture be applied to ion-target collisional activation? A study of keV CO2+*/He collisions by emission spectroscopy

Collision-induced photon emissions (CIE) were observed for keV CO(2)(+*)/He collisions from 190 to 1020 nm. The emissions were assigned to the Deltanu=0 band of the CO(2)(+*) B (2)Sigma(u)(+) --> X (2)Pi(g) electronic transition and the Deltanu= +3, +2, +1, 0, -1, -2, -3 vibrational transition progression in the CO(2)(+*) A (2)Pi(u) --> X (2)Pi(g) electronic transition. The other peaks arise from the emissions of excited O* fragment atoms and the target gas. The relative intensities of the CO(2)(+*) and O* emissions are independent of the ion translational energy above 3 keV, supporting the curve-crossing mechanism for collisional excitation. Investigation of the relative intensities within the A (2)Pi(u) --> X (2)Pi(g) emission of CO(2)(+*) indicates that the vibrational distribution is well described by the Franck-Condon principle at high collision energy, a consequence of short collision time but not necessarily an indication of vertical transitions. Below 3 keV ion translational energy, vibrational excitation in the A (2)Pi(u) electronic state was observed. The observation is consistent with the explanation that the reaction occurs at small impact parameters, in which short-range, repulsive interactions between the projectile and the target result in direct translational-vibrational excitation.

Experimental Evidence for the Curve Crossing Mechanism for Collisional Excitation in keV N2+•/He Collisions by Emission Spectroscopy

Collision-induced emission (CIE) experiments were carried out by coupling a spectrograph and charge-coupled device detector (CCD) to a commercial analytical mass spectrometer. An Einzel lens and a deceleration-reacceleration lens assembly as described in the current article were installed in the mass spectrometer to allow for the deceleration of the ions before collision. Collision-induced emission spectra of N2+*/He collisions at lab frame collision energies from 2 to 8 keV were obtained from 190-1020 nm. The emissions were assigned to the Deltav=+2, +1, 0, -1, -2 vibrational transition progression in the N2+* B 2Sigmau+-->X 2Sigmag+ electronic transition as well as some atomic lines from the fragments N+, N* and the target gas He. N2+* A 2u-->X 2Sigmag+ emission was also observed but was very weak due to the long lifetime of the A 2u state. The relative intensities of the N2+*, N, and N+ emissions are independent of the ion translational energy within the studied energy range. This observation supports the curve-crossing mechanism for collisional excitation, suggesting that a complicated sequence of curve-crossings takes place upon collisional activation.