Stereo-Dynamics of Autoionization Reactions Induced by Ne*(3P0,2) Metastable Atoms with HCl and HBr Molecules: Experimental and Theoretical Study of the Reactivity Through Selective Collisional Angular Cones

In this paper are presented mass spectrometric determinations as a function of the collision energy in the 0.03–0.50 eV range as recorded in a crossed molecular beam experiments involving autoionization reactions between Ne*(3P2,0) metastable atoms and HCl and HBr molecules. The total and partial ionization cross sections for both investigated systems are presented and discussed in a comparative way. The comparison of the recorded data allows to point out similarities and differences on the collisional stereodynamics of Ne*(3P0,2)-HCl and Ne*(3P0,2)-HBr systems. In particular, an accurate characterization of the interaction potentials, which is mandatory for a comprehensive description of Ne*-HX (X = Cl and Br) reactive collisions, has been outlined. Such a theoretical analysis suggests that the formation of the proton transfer, NeH+, ions as well as of other possible product ions (i.e. HX+ and NeHX+, parent and associate ions, respectively) comes from reactivity that is selectively open along angular cones showing different orientation and acceptance. In particular, the performed analysis highlights that the proton transfer rearrangement reaction, which is open in both Ne*-HX autoionizing collision, is much more efficient for Ne*+HCl respect to Ne*+HBr autoionization. The present investigation points out that such an efficiency variation is related to the following crucial points: (i) the different charge distribution on HX+ ionic products, (ii) the balance between two distinct microscopic mechanisms that are operative in such processes (a pure physical-photoionization-indirect mechanism and a chemical-oxidation-direct mechanism), which are reactions of interest in combustion chemistry, plasma physics and chemistry, as well as in astrochemistry and for the chemistry of planetary ionospheres.