Charge exchange ionization in collision cells as a method to detect the presence of long-lived excited electronic states of polyatomic ions

Charge exchange ionization in collision cells installed in a double focusing mass spectrometer with reversed geometry has been used to detect the presence of a long-lived excited electronic state of benzene ion. In particular, the first collision cell located between the ion source and the magnetic sector was modified to serve as an ion source for the reagent ion generated by charge exchange with the primary ion. Strong reagent ion signals were observed when the ionization energies of the reagents (1,3-C₄H₆, CS₂, CH₃Cl) were lower than the recombination energy (∼11.5 eV) of the excited state benzene ion, while the signals were negligible for reagents (CH₃F, CH₄) with higher ionization energy. The fact that a strong signal is observable only for electronically exoergic charge exchange is useful for detecting the presence of a long-lived electronically excited state.