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 C 6 H 5 +, C 6 H 6 +, C 6 H 7 +, C 12 H 12 +, C 12 H 11 +, naphthalene (C 10 H 8 + ) cation, and biphenyl (C 12 H 10 + ) 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 (C 12 H 12 + and C 12 H 11 + ) 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.