EXCITATION MECHANISMS IN MODERATE-ENERGY NA-AR COLLISIONS()

By means of differential energy-transfer measurements, excitation mech anisms in Na+-Ar collisions have been studied at laboratory collision energies of 50 less than or equal to E(lab)less than or equal to 1500 eV. In these experiments, doubly differential cross sections have been measured over nearly the whole angular range in the center-of-mass sy stem by simultaneously detecting both scattered and recoiled particles (Na+, Na, Ar+, and Ar) at laboratory angles of 2 degrees less than or equal to theta less than or equal to 92 degrees. The Na+ ions and Na atoms scattered inelastically were observed at reduced angles of tau>3 .5 keV deg and collision energies of E(lab)>500 eV. For energies of 50 0<E(lab)<1000 eV, the dominant inelastic signal is due to one-electron charge transfer, while for E(lab)>1000 eV, two-electron excitation as well as one-electron excitation were observed. The electronic transit ions in the Na+-Ar collisions are classified into two types of excitat ion mechanisms. One type is the one-electron transition that takes pla ce at internuclear distances of R<R(C1) = 1.07 Angstrom. The other typ e is the one- and two-electron transitions that-occur at distances of R<R(C2)=0.45 Angstrom. These critical distances R(C)=1.07 and 0.45 Ang strom are close to \r(i)+/-r(j)\=0.96 and 0.38 Angstrom, respectively, evaluated from the ionic and atomic radii.