Determination of rotational relaxation rates by measurement of the time dependence of saturation in microwave spectroscopy

A spectrometer for measuring the time dependence of a microwave transition is described. The instrument uses the Stark effect to turn the absorption on and off, and an electronic integrator to obtain the desired signal‐to‐noise ratio. The qualitative features of the time‐dependent spectra as a function of sample pressure, microwave power, and microwave frequency are described. A numerical procedure, based on an equation previously derived [J. Chem. Phys. 57, 3511 (1972)] for extracting the pressure‐dependent relaxation rate is outlined. The results of application of the method to the determination of relaxation rates for the $\text{J = 1 ← 0}$ and $\text{J = 2 ← 1}$ transitions in OCS, and the J=3, K=3 inversion transition in NH₃ are detailed. In all three cases it is found that the pressure‐dependent relaxation rates are very similar to those implied by linewidth measurements. The implications of this result are discussed.