Multiple-electron removal and molecular fragmentation of CO by fast F4+ impact.

Abstract

Multiple-electron removal from and molecular fragmentation of carbon monoxide molecules caused by collisions with 1-MeV/amu ${\mathrm{F}}^{4+}$ ions were studied using the coincidence time-of-flight technique. In these collisions, multiple-electron removal of the target molecule is a dominant process. Cross sections for the different levels of ionization of the CO molecule during the collision were determined. The relative cross sections of ionization decrease with increasing number of electrons removed in a similar way as seen in atomic targets. This behavior is in agreement with a two-step mechanism, where first the molecule is ionized by a Franck-Condon ionization and then the molecular ion dissociates. Most of the highly charged intermediate states of the molecule dissociate rapidly. Only ${\mathrm{CO}}^{+}$ and ${\mathrm{CO}}^{2+}$ molecular ions have been seen to survive long enough to be detected as molecular ions. The relative cross sections for the different breakup channels were evaluated for collisions in which the molecule broke into two charged fragments as well as for collisions where only a single charged molecular ion or fragment were produced. The average charge state of each fragment resulting from ${\mathrm{CO}}^{\mathit{Q}+}$\ensuremath{\rightarrow}${\mathrm{C}}^{\mathit{i}+}$+${\mathrm{O}}^{\mathit{j}+}$ breakup increases with the number of electrons removed from the molecule approximately following the relationship i\ifmmode\bar\else\textasciimacron\fi{}=j\ifmmode\bar\else\textasciimacron\fi{}=Q/2 as long as K-shell electrons are not removed. This does not mean that the charge-state distribution is exactly symmetric, as, in general, removing electrons from the carbon fragment is slightly more likely than removing electrons from the oxygen due to the difference in binding energy. The cross sections for molecular breakup into a charged fragment and a neutral fragment drop rapidly with an increasing number of electrons removed.