Methane as Carrier Gas for Molecular Beam Microwave Spectroscopy to Observe Transitions of Higher Rotational Energy Levels

The combination of Fourier transform microwave (FTMW) spectroscopy with the technique of pulsed supersonic molecular beams (MB) by Balle and Flygare [1] resulted in a great improvement of rotational spectroscopy [2]. One advantage of the beam technique is the simplification of the rotational spectra observed in a static gas, which often, especially for asymmetric top molecules, are very dense by numerous rotational transitions in different vibrational states. Only transitions originating from low rotational energy levels are left, since the collisional cooling in the early stages of the beam expansion depopulates higher energy levels. Thus, the initial assignment of the spectra also of rare isotopomers is much easier. This advantage converts to a disadvantage, if one is interested in effects produced by centrifugal distortion or methyl internal rotation. Here, the effects are more pronounced in high J transitions. It should be mentioned that the higher resolution [3] and precision of MB F T M W spectroscopy reveals these effects, compared to spectroscopy on static gases, already for lower J transitions. Nevertheless, the study of small effects still requires the observation for higher J transitions. Recently, we were faced to investigate the centrifugal distortion in our effort to study the hyperfine structure of 3 S in thiirane, C H , C H , [41. • L J S