Rotational temperature dependences of gas phase ion–molecule reactions

Abstract

A technique for measuring the rotational temperature dependences of gas phase ion–molecule rate constants is presented. The technique involves measuring the kinetic energy dependences of the rate constants at several temperatures in a variable temperature selected ion flow drift tube. For a monatomic ion, comparing the rate constants at the same center of mass kinetic energy at different temperatures yields the dependence of the rate constant on the internal temperature of the reactant neutral. For neutrals in which the vibrational modes are inactive at the temperatures of the experiment, the internal energy dependence is the rotational temperature dependence. Two examples are presented here, one in which rotational energy significantly influences the rate constants, approximately T−0.5, and one in which it does not. Implications for past drift tube experiments are discussed.