Charge and energy transfer in symmetric doubly chargedAr2++Ar collisions

This article describes a measurement of charge- and energy-transfer reactions in ion-atom collisions involving a doubly charged projectile ion. The reactions studied, ${\mathrm{Ar}}^{2+}$ ${(}^1$S, ${}^1$D, ${}^3$P)+Ar, are important in nonreactive plasmas and in chemical plasma processes like those used to deposit metal oxides and in plasma etching. This study reports the measurement of the transfer of charge and electronic energy from the metastable ${}_{}{}^1{}_{}{}^{}\mathrm{S}$ and ${}_{}{}^1{}_{}{}^{}\mathrm{D}$ states of ${\mathrm{Ar}}^{2+}$ to the parent atom. The experiment uses a novel technique in which the angular deflection and energy of a near-thermal target atom produced by expansion from a supersonic jet is measured. The metastable reactions ${(}^1$S,${}^1$D→${}^3$P) result from collisions in which the particles penetrate to a small distance of closest approach and undergo a relatively large angle of deflection. The symmetric charge transfer which occurs in the ground ${(}^3$P→${\mathrm{}}^3$P) state of the doubly charged system yields a large number of oscillations in scattering cross section whose peaks are separated by 104 eV deg for collisions of 260 eV primary impact energy. These oscillations are caused by the ungerade-gerade character of the interaction, that is, they result from interference resulting from the symmetric and antisymmetric interaction potentials. This study enables the measurement of both the endothermic and exothermic energy-transfer reactions in which electronic and ion kinetic energy is interchanged.