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Abstract.We report accurate laboratory measurements of selected methanol transition frequencies between 0.834 and 230 GHz in order to facilitate astronomical velocity analyses. New data have been obtained between 10 and 27 GHz and between 60 and 119 GHz. Emphasis has been put on known or potential interstellar maser lines as well as on transitions suitable for the investigation of cold dark clouds. Because of the narrow line widths (<0.5 km s −1 ) of maser lines and lines detected in dark molecular clouds, accurate frequencies are needed for comparison of the velocities of different methanol lines with each other as well as with lines from other species. In particular, frequencies for a comprehensive set of transitions are given which, because of their low energy levels (<20 cm −1 or 30 K), are potentially detectable in cold clouds. Global Hamiltonian fits generally do not yet yield the required accuracy. Additionally, we report transition frequencies for other lines that may be used to test and to improve existing Hamiltonian models.

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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Some disadvantages of using the Fouriertransform algorithm to obtain rotational spectra from microwave pulse induced transient signals are demonstrated. To overcome the difficulties, a fitting procedure is suggested which is based on the analysis of time-resolved data.