3D VMI Of Low-energy Ion-Molecule Reaction And Experimental Study On The Reaction Dynamics Of Ar⁺ With CO₂ And O₂

The ion-molecule reaction is frequently observed in interstellar space,planetary atmosphere,combustion flames,and plasma etching,and play an important role in their material evolution.Its dynamics research helps us to establish these material evolution models more accurate,which is of great significance in relevant research or practical application.For a century,experimental methods have been continuously improved with the development of technology.Due to the development of supersonic molecular beam technology and particle detection technology,at present,the main method for ion-molecular reaction research is the cross beam method with weak signal but most dynamic information.Many gratifying results have been obtained in the field of reaction dynamics with cross beam research,and the related theories have been developed accordingly.Our group has also done a lot of work in the field of ion-molecular reaction.Firstly,to realize the direct detection of the full three-dimensional Newton sphere,the detection system of our low-energy ion-molecule reaction velocity map imaging device was updated.We have redesigned the lens suitable for three-dimensional velocity map imaging and upgraded the detector from a phosphor screen+CCD camera to a delay-line anode detector so that the entire detection system has the capability of full three-dimensional direct imaging.Internationally,for the first time,we have introduced direct full-three-dimensional ion velocity imaging technology into low-energy ion-molecule reaction experiments,which will bring innovations to the study of stereodynamics.owing to the upgrade of the detector,the use of high pulse voltage in time slices is avoided and improves the repetition rate of the experiment,which greatly improves the efficiency of the experiment.We successfully recovered the full three-dimensional Newton sphere of ion products using a self-developed data extraction package and wrote the subsequent data processing and analysis program for different reaction systems.With the upgraded instrument,we have carried out the following experiments.(1)In a center-of-mass collision energy range of 7.23 eV～15.96 eV,we studied the charge transfer reaction between Ar+and CJ2.After analysis,we found that CO2+products are primarily populated at the ground state X2Πg and the single-electron excited states A2Πu,B2Σu+ and C2Σg+;the multielectrou excited states of CO2+ are also found at the higher collision energies.The production efficiency profiles of CO2+ are distinctly different from the photoionization electron spectrum of CO2,implying that the charge transfer from Ar+would be not as fast as expected.The strong electron correlations in the short-lived intermediate(Ar-CO2)+should be responsible for the CO2+ yields at the multielectron excited states.(2)The charge transfer reactions between Ar+(2P)and O2(X3Σg-)are investigated in the collision energy range of 3.40 eV-9.24 eV within the center-of-mass coordinate,by using the ion momentum imaging technique.The internal energy of the product O2+is enhanced gradually with the increase of collision energy,and the forward-scattered O2+ ions are distributed in the broader range of scattering angles at higher collision energies.At the low collision energy of 3.40 eV,the resonant charge transfer,similar to a photon ionization process,leads to the Franck-Condon-like vibrational state population of O2+ at the a4Πu state.At the higher collision energies,besides a4Πu and the high-lying states that are visible in the photoionization process,the O2 products could be populated at some electronically bound states in the non-Franck-Condon region.The present observations indicate again the strong collision-energy dependences of the charge exchange reactions,but distinctly different from our previous finding for Ar++NO→Ar+NO+.(3)Stereodynamics of molecular collisional reaction arising from the mutual alignment or orientation between the reactants has been an attractive topic in chemistry for decades.However,co-existence of the collinear and no-collinear collisions was never observed experimentally,furthermore,on our common sense,the stereodynamics aspects are usually absent for the randomly orientated reactants.We studied the dissociation charge transfer process of Ar+and O2,here between randomly orientated O2 and low-energy ion Ar+,we,using three-dimensional ion velocity-map imaging technique,clearly observed a linear alignment and a nearly isotropic distribution of the O+yields along the collision axis.Above observations,as well as their corresponding energy-transfer efficiencies,are interpreted with two Doppler dynamics models and further elucidated with the spin-state couplings of O2+,indicating the remarkable charge-and energy-transfer preferences in the collinear and no-collinear collisions.