Molecular Dynamic Studies of the Charge-Transfer Reaction by the Crossed-Molecule-Ionic Beam Method

The basic mechanisms for low energy electron transfer processes have beena subject of much debate for several years. The role of energy resonance andFranck-Condon factors, the presence or absence of intersecting potential ener-gy surfaces for these processes and the possible participation of collision com-plexes have been discussed extensively. The reactions of rare gas ions withsimple molecules have been studied most extensively as a source of informationon these issues, largely because these systems have well defined and relativelywidely spaced energy levels. Charge-transfer (CT) reactions are also a suitable subject for energy con-version investigations between chemical particles because they provide favorableopportunities to examine the fate of translational and internal energy in anelementary process. Using an apparatus described in detail elsewhere, complete dynamical stu-dies of the differential cross-sections as a function of collision energy and scat-tering angle have been carried out for the reactions Ar~+(Ar,Ar)Ar~+, Kr~+(Kr,Kr)Kr~+, N_2~+(N_2,N_2) N_2~+, CO~+(CO,CO)CO~+, Ar~+(N_2,Ar)N_2~+, Ar~+(CO,Ar)CO~+,Ar~+(NO, Ar) NO~+, Ne~+(N_2, Ne) N~+ and Ne~+(N_2, Ne~+N)N~+ in the rangeof collision energies from below lev to 20ev (center-of-mass). The charge transfer reaction of Ar~+ with Ar provides a very clear exampleof translationally resonant charge transfer (TRCT) over the entire range oftranslational energies investigated. The reaction of N_2~+ with N_2 at low collision energy exhibits two mecha-nisms, a direct resonant CT mechanism and one involving extensive energytransfer. The angular scattering diagram for this reaction at 0.7ev collisionenergy evidence for two well-resolved mechanisms-a direct mechanism which N_2~+(X~2∑g,V=0)+N_2~+(X~1∑g,V=0)→N_2~+(X~2∑g,V′)+N_2(X~1∑g,V")may be described qualitatively as an electron jump mechanism and a "comp-lex" mechanism distributing products symmetrically about the center of mass. The charge transfer reaction of Ar~+ with N_2 is highly quantum state spe-cific. It is properly written as Ar~+(~2P_(3/2))+N_2(~1∑g,V=0)→N_2~+(~2∑g,V=1)+ Ar(~1So)A long-range curve-crossing mechanism iuvolving the coupling of adiabaticcurves for Ar~+N_2 with N_2~+Ar as described by Spalburg and Gislason properlyaccounts for the large cross section, specific population of the V=1 level andthe observed small angle scattering at high energy.