Ionization and Fragmentation of Molecular Gases in Collisions with Mev/amu Heavy Ions.

A detailed study of ionization and fragmentation in molecular gas targets by 2.75 MeV H^ {+} and 40 MeV Ar^{13+ } ions has been performed using the time-of -flight (TOF) method. Ionization of molecular gases by 2.75 MeV H^{+} ions results primarily in single and double ionization, and consequently, the fragmentation patterns are simple to interpret. Energetic highly-charged, heavy ions, such as 40 MeV Ar^ {13+}, were found to be very effective in producing multiple ionization in molecular gas targets either through direct ionization or electron capture. Multiply -charged molecular ions, which are typically very unstable, rapidly dissociate to yield a rich fragmentation pattern containing numerous atomic and molecular fragments. A two stage TOF mass spectrometer was designed, constructed and tested for application to the study of fragmentation in molecular gas targets. The TOF spectrometer was calibrated and optimized for resolution and space focussing using neon and argon gas targets. To verify the proper working conditions for the spectrometer, several experiments were performed using noble gas targets including coincidence measurements of the recoil ion charge state distributions with the post-collision charge states of a 40 MeV Ar ^{13+} beam. Partial cross sections for direct ionization, one-, two- and three-electron capture in neon and argon by 40 MeV Ar^{13+ } ions have been determined and compared to ion-atom collisions theories. The fragmentation patterns for several molecular gases from simple diatomics (N_2, O_2, CO, NO) and triatomics (CO _2, N_2O, NO_2, SO_2) to large polyatomics (C_2F _6, C_2H_4 F_2, SF_6 ) have been measured. The fragmentation patterns for molecular gases measured by the TOF method were found to be useful in providing information about the various pathways for unimolecular dissociation of multiply-charged molecular ions. Moreover, several useful quantities such as fragments yields and average kinetic energies were obtained from the TOF spectrum. Energetic fragments produced in the dissociation of a multiply-charged molecular ion were found to exhibit a characteristic split peak structure in the TOF spectrum, whereas less energetic fragments, such as central atoms in totally symmetric molecules, had very narrow peak widths in the TOF spectrum similar to those observed for noble gas recoil ions. A simple model is proposed to predict the lowest energy pre-dissociative state in multiply-charged molecular ions.