MOBILITIES OF AROMATIC IONS DRIFTING IN HELIUM

Aromatic ions are produced in an electron impact ion source and inject ed into the flow tube section of a selected-ion flow-drift tube (SIFDT ) apparatus. The reduced mobilities of C6H5+, C6H6+, C6H7+, C12H12+ C1 2H11+, naphthalene (C10H8+) cation, and biphenyl (C12H10+) cation drif ting in helium are measured as a function of the characteristic parame ter E/N (the ratio of the electric field to the number density of the helium buffer gas atoms in the drift tube). The reduced mobilities of the benzene dimer ions (C12H12+ and C12H11+) are compared with the mea sured mobilities of naphthalene and biphenyl cations to correlate the mobilities with ionic structures. An angle-averaged collision cross se ction of the ions drifting in helium is obtained from the geometric st ructure of these ions and is used to compute the zero-field mobility. The predicted zero-field mobilities are found to compare well with the experimentally measured mobilities.