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 pressurehyphen;dependent sources, and give experimental results for these transitions: OCSJ=0minus;1 andJ1ndash;2, H2CO 212minus;211, NH3(3,2), and ethylene oxide 431minus;422. Previous transient experiments need to be rehyphen;evaluated in terms of a speed dependent relaxation rate. The speed dependence may make the measured rate an illhyphen;defined population average which cannot be compared to static line width measurements or to calculations.
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