Mobilities of aromatic ions drifting in helium

Abstract

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.