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

Abstract

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 J = 1 ← 0 and 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.