M Krishnamurthy, J A. de Gouw, V M. Bierbaum, S R. Leone
Benzene dimer cations (C12H12+and C12H11+) are produced in the flow tube section of a selected-ion flow-drift tube (SIFDT) apparatus. The reduced mobilities of these ions in helium are measured as a function of the characteristic parameter E/N (the ratio of the electric field to the number density of the helium buffer gas atoms in the drift tube). The reduced mobilites of the dimer ions are compared with the measured mobilities of naphthalene and biphenyl cations to correlate the mobilities and structures. The hard sphere cross sections calculated from the experimentally determined mobilities in the zero field limit, using momentum transfer theory, compare very well with the hard sphere cross sections computed from the geometric structures of ions using a hard sphere model. The results suggest that the sandwich structure is slightly favored for C12H12+ ions. Additional results on the reduced mobilities of C6H5+ and C6H7+ are also obtained.