Delayed dissociation spectra of survivor ions from high-energy collisional activation

Collisional activation (CA) of large ions at kiloelectronvolt energies is accompanied by unexpectedly large losses of translational energy, which vary with the nature of the collision gas. Previous investigations have concentrated upon subsequent fragmentations occurring within a time window covering a few fis immediately following collision, using massanalyzed ion kinetic energy spectrometry. In the present work, survivor ions were selected for specified values of translational energy loss, and their internal energy contents assessed via their subsequent unimolecular fragmentation reactions within a later time window. Beam collimation was also applied when circumstances permitted to impose angular selection, thus minimizing cross talk between effects of collisional scattering and energy dispersion. It was shown that internal excitation of the reactant ion can account for only a small fraction of the observed loss of translational energy. The recoil energy of the target is thus the principal sink for the translational energy loss, since the latter was always chosen to be less than the lowest excitation energy of the target. This conclusion is shown to be consistent with theoretical models of the CA process. The practical implications of these conclusions for CA of large ions at kiloelectronvolt energies are discussed.