A tandem mass spectrometry study of the role of electronically excited states in the collision-induced dissociation of the hexafluorobenzene molecular cation

Dissociation of hexafluorobenzene cations via C-F bond cleavage following c ollisional activation with helium and argon has been studied at 1-4 keV (no minal) laboratory kinetic energies, When hexafluorobenzene cations are form ed by 70 eV electrons, the kinetic energy distributions of the fragment ion , C6F5+, differ dramatically in that helium collisions proceed via highly e ndothermic channel whereas argon collisions result in a highly exothermic c hannel. Both processes are accompanied by a common dissociation pathway, wh ich is nearly thermoneutral. With low energy electron ionization of C6F6 an d argon collision gas the superelastic peak disappears and an endothermic p rocess analogous to that resulting from helium collisional activation appea rs. These experimental observations suggest that 70 eV electrons result in the formation of hexafluorobenzene cations in electronically excited state( s) that have radiative lifetimes exceeding 20 mu s. The exothermic process corresponds to energy release of similar to 10 eV into translational mode d uring the collision and the endothermic process corresponds to transfer of similar to 17 eV energy from translational to internal modes. Since the thr eshold for dissociation is 7 eV, we suggest that both endothermic and exoth ermic channels proceed via the same electronically excited hypersurface. (I nt J Mass Spectrom 194 (2000) 171-179) (C) 2000 Elsevier Science B.V.