| Icn-molecule reaction is one of the most fundamental processes in the Earth and other planets’ atmosphere,interstellar space,and combustion,its cross section is much higher than the atom-molecule reaction.However,with respect to the atom-molecule collisions,the ion-molecule reaction still needs more attention.Charge exchange,energy transfer,dissociation and combination are frequently involved in the low-energy(less dozens of eV)ion-molecule reactions.Measurements of the momentum(kinetic energy and angular)distributions of the ionic products can provide us the dynamics details at quantum state level.Therefore,we decide to set up a velocity map imaging(VMI)crossed-beam apparatus for the experimental study of this topic.At first,we build a pulsed low-energy ion beam source,in which the ions produced by the pulsed electron impact are confined well in the spatial size of each bunch.Besides the ion focusing method to reduce the transverse section of the beam,the longitudinal section in the translational direction is compressed by introducing a second pulse in the ion time-of-flight system.The test experiments for the low-energy argon ions are performed,and it is proved that this beam source is appropriate for applications in the ion-molecule reaction dynamics experiments,in particular,in combination with the ion VMI technique.Secondly,in combination of this well-confined ion beam source and ion VMI technique,we establish a crossed-beam apparatus in which the reactants are the neutral molecules produced from a skimmed supersonic beam and the ions coming from the well-confined ion source,and the product ions are detected with the time-sliced VMI system.The efficient performance of this apparatus is evaluated by measuring the momentum distributions of N2+/NO+ after the collision between Ar+ and N2/NO.In the charge exchange reaction,the high-lying quantum states of products are apt to be populated by translational-to-internal energy transfer in collisions,which becomes more efficient with the increase of collision energy.However,in the reaction of Ar+ +NO→NO++Ar,we find that the NO+ products prefer to populate at the lowest triplet state a3∑+,particularly in the higher energy collisions;the higher states b3Πand w3△ of NO+ are accessed only at the lower collision energies.Such striking collision-energy dependence is attributed to two distinctly different processes:the former is controlled with an energetically resonant charge-transfer mechanism;while an intermediate complex(Ar—NO)+ formed in the latter process permits the nonadiabatic energy redistributions and leads to several energetically close states of NO+.In the charge transfer experiments for Ar+ and N2 in relative collision energy range of 2～10eV,we find that the product N2+ at the higher collision energy exhibits the distribution of electronic excited states A2Πu and B2∑u+ besides the distribution of ground state X2∑g+ with the vibrational energy level v’ = 0-3.N2+(X2∑g+)at higher v’ level is scattered into the larger scattering angle,that conforms the typical feature of Landau-Zener curve crossing mechanism.The vibrational state distributions of the A-state N2+ can be attributed to the curve crossings of X—A and A—B states,which is the result of the ’intensity borrowing’ induced by vibronic coupling.This is a model system of non-Born-Oppenheimer reaction mechanism,the energy distribution of N2+ is controlled by collision partners’ behaviors around the conical intersections between different potential energy surfaces,instead of Franck-Condon principle. |
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