High-energy collisional activation studied via angle-resolved translational energy spectra of survivor ions

Angle-resolved translational energy spectroscopy has been applied to Cs4I + (3) ions that survived 8 keV collisions with a range of collision gas targets, including inert gases and deuterium. The experimental data comprise values of the translational energy loss ΔTR as a function of the (laboratory-frame) scattering angle θ R for each collision gas under conditions such that single-collision events dominated the scattering. The values of ΔTR increase with θ R, in accordance with very general expectations. However for any value of θ R, the values of ΔTR for helium and deuterium as targets were almost indistinguishable from one another but were at least five to six times larger than those for neon and all other collision gases. These data have been shown to be consistent with theoretical considerations based upon conservation of energy and linear momentum. Theoretical approaches include the simple "elasticlimit" model, which makes no mechanistic assumptions, and a particular "binary-model" theory, which excludes electronic excitation as a possibility. Both theories are consistent with the experimental data and interpret the surprisingly large values of ΔTR for low-mass targets in terms of large recoil energies of the target required to ensure conservation of momentum. The most likely alternative candidate as sink for ΔTR is internal excitation of the target, but this possibility was excluded in the present work by choosing ΔTR values less than the lowest excitation energies of the inert gas targets. Moreover, such an interpretation cannot explain the similar results obtained using helium and deuterium, which were markedly different from those obtained for all other collision gases.