C-I and C-F bond-breaking dynamics in the dissociative electron ionization of CF3I.

Abstract

We present a comprehensive experimental study into the dissociative electron ionization dynamics of CF3I at energies ranging from 20 to 100 eV. A beam-gas instrument has been used to measure the absolute total ionization cross-section for the molecule over the energy range from 0 to 300 eV. Coupled with data from an electron-molecule crossed beam velocity-map imaging instrument, this allows absolute partial ionization cross-sections to be determined for formation of each ionic fragment. These reveal a number of fragmentation channels involving both C-I and C-F bond cleavage, in some cases followed by further fragmentation of the resulting molecular ion. Velocity-map images have been recorded for the I+ and CF3+ products of C-I bond cleavage and the CF2I+ products of C-F bond cleavage. Analysis of fragment kinetic energy distributions extracted from the images reveals that CF3+ product of C-I bond cleavage appears to be formed via a statistical mechanism occurring over long timescales, while the CF2I+ products of C-F cleavage are formed via a much faster, more direct dissociation mechanism involving one or more repulsive states of the parent molecular ion. The I+ fragments arising from C-I bond cleavage display behaviour intermediate between the two extremes. For all fragments, the images show little or no dependence on the energy of the incident electron, implying that the initially excited ion state or states undergo rapid relaxation to the dissociative state(s) in all cases. Only a very small fraction of the incident electron's kinetic energy is released into kinetic energy of the recoiling atomic and molecular fragments, implying that most of the available energy remains with the two departing electrons. The kinetic energy distributions obtained for the various fragments of dissociative electron ionization are compared with the corresponding distributions reported from photoionization studies in order to gain insight into the electronic states involved.