Mobilities of aromatic ions drifting in helium

Aromatic ions are produced in an electron impact ion source and injected into the flow tube section of a selected-ion flow-drift tube (SIFDT) apparatus. The reduced mobilities of C6H5+, C6H6+, C6H7+, C12H12+ C12H11+, naphthalene (C10H8+) cation, and biphenyl (C12H10+) cation drifting 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 mobilities of the benzene dimer ions (C12H12+ and C12H11+) are compared with the measured mobilities of naphthalene and biphenyl cations to correlate the mobilities with ionic structures. An angle-averaged collision cross section of the ions drifting in helium is obtained from the geometric structure 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.