Speed dependence of microwave rotational relaxation rates

Abstract

For most molecular systems, rotational relaxation rates depend strongly on absorber speed. The influence of this speed dependence on transient decays in the microwave region and on the measurement of rotational relaxation rates, line positions, and line shifts is described. Measurement of the speed dependence of relaxation rates may be made from the nonexponential behavior of transient decays which are more sensitive to the speed dependence than line shapes. We describe how these transient experiments may be performed using either amplitude modulation or Stark switching, how the data may be analyzed using calculations of nonexponentiality from pressure-dependent sources, and give experimental results for these transitions: OCS J=0−1 and J1–2, H2CO 212−211, NH3 (3,2), and ethylene oxide 431−422. Previous transient experiments need to be re-evaluated in terms of a speed dependent relaxation rate. The speed dependence may make the measured rate an ill-defined population average which cannot be compared to static line width measurements or to calculations.