Flowing Afterglow Studies of Ion–Molecule Association Reactions

Positive and negative ion–molecule association reactions have been investigated in a flowing afterglow system at 82° and 280°K with helium as a third body over a pressure range 0.1–3.0 torr. Specific rates have been measured for the association reactions Ar++Ar, O++N2, N++N2, O2++O2, N2++N2, and O−+CO2. At low pressures the kinetics of association were termolecular as expected for the ground-state ions studied. The termolecular rate coefficients were found to decrease significantly with increasing temperature. At 82°K the kinetics of association for both O2++O2 and N2++N2 become bimolecular above helium pressures of about 1.6 and 0.2 torr, respectively. This observation is qualitatively, but not quantitatively, consistent with the simple mechanism usually invoked for association reactions. A consideration of available laboratory data on ion–neutral termolecular associations in which a stable complex is formed indicates a surprisingly systematic variation of the termolecular rate coefficient with the number of atoms in the product-ion cluster. The cluster ions N6+, N7+, N8+, N9+, and Ar3+ were identified mass spectrometrically at 82°K, apparently for the first time.