Kinetic Studies of Negative Ion Reactions in a Quadrupole Ion Trap: Absolute Rate Coefficients and Ion Energies

A new apparatus consisting of an ion−molecule reactor coupled to a quadrupole ion trap is described. The kinetics and products of the rapid reactions of SF 6 - with SO 2, HNO 3, and HCl in the ion trap are reported. There is excellent agreement between the ion trap rate coefficients and yields, and flowing afterglow results. Rate coefficients for reactions of products of the SF 6 - reactions are also reported. Measurements of the equilibrium constant for the reaction NO 3 - ·HNO 3 + HNO 3 ⇄ NO 3 - ·(HNO 3 ) 2 in the ion trap provide a very sensitive measure of the energy of the trapped ions. These measurements indicate that the effective temperature of the trapped NO 3 - ·(HNO 3 ) 2 ion is 320 ± 30 K for an ion trap at 298 K (1−2 mTorr He, q z = 0.074). The presence of 0.2 mTorr N 2 buffer gas increases the internal temperature of the trapped ions by about 10 K. The internal temperature is also a weak function of the trapping potential, increasing by about 10 K over the range 0.074 < q z < 0.286.