The dependence of rotational relaxation rates on the speed of absorbing molecules has been studied by millimeter wave coherent transients for the J,K=1,1–2,1 rotational transition of methyl fluoride (CH3F). A new phenomenological model used to describe such a speed dependence has been introduced. It leads to a quite simple analytical expression for time-domain transient signals, the Fourier transform of which corresponds to the frequency-domain line shape (speed-dependent Voigt profile). The investigations were carried out on mixtures of CH3F with He, Ar, Xe, H2, D2, N2, and O2, yielding parameters which characterize the speed dependence of the observed decay rates and its pressure and temperature dependence. Special emphasis was given to the key role of the mass ratio of collision partners which clearly allowed the relation of the observed nonexponential decay behavior to collisional effects. However, the observations cannot be explained exclusively with consideration of speed-dependent rates, but must also be discussed with reference to velocity-changing collisions. The observed temperature dependence of the rates may allow discrimination between these two different collisional effects which lead to departures from Voigt profile line shapes.

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Time domain techniques using pulse methods in microwave spectroscopy have been employed to determine the population and coherence decay rates, l / T~ and l/T2, for the J = 0-1 rotational transition of ~sNzO. The pressure dependence of both relaxation rates has been investigated for the pure gas. Pressure broadening of the line (,-, l/Tz) has also been studied for mixtures with 02, N2, air and the rare gases He, Ne, Ar, Kr and Xe. The observed transient emission signals were analysed by the standard model of constant coherence decay rates and also assuming dependence of the rates on the speed of the absorber molecules. The experimental results are compared with predictions from theoretical treatments of the binary collisions.

The microwave spectrum of 4-methylisothiazole was assigned by means of a pulsed molecular beam microwave Fourier transform spectrometer. Analysis of the spectrum yielded the rotational constants A = 7370.642 (