The design, performance, and operation of a broadband (3–26.5 GHz) high resolution microwave spectrometer is described. In comparison to previously developed molecular beam Fabry–Perot resonator spectrometers the design presented here implements some significant improvements: a coaxially oriented beam resonator arrangement (COBRA) formed by a confocal pair of mirrors incorporating an electromechanical valve and employing two pairs of microwave antennas, and a multioctave Fourier-transform microwave (FTMW) instrument providing the pulsed excitation source with microwave pulse phase-inversion scheme and the low-noise receiving system employing image-rejection downconversion with superheterodyne as well as quadrature detection. The entire apparatus, fully automated for scanning operation, covers a frequency range of more than three octaves. The novel design of the FTMW instrument does not require any changes of the spectrometer hardware in order to reach all regions of its spectral range. While operated in high resolution mode the COBRA-FTMW spectrometer is achieving a linewidth (half-width at half-height, HWHH) relative to the molecular emission frequency of better than 10−7 HWHH. The sensitivity and resolution of the spectrometer is demonstrated by several examples.

The microwave spectrum of trans-2,3-dimethyloxirane (CH 3 CHOCHCH 3 ) in the excited torsional states u 17 = 1 and v i3 -1 has been measured in the range from 8 to 26 GHz and assigned. An analysis of internal rotation splittings of the observed rotational transitions was performed using the internal axis method (or "combined axis method") with a newly developed program accounting for the top-top coupling. The threefold hindering potential V 3 and the direction cosines /. g of the internal rotation axes i with respect to the principal inertia axes g are in a good agreement with the ground state values. Additionally, the sixfold hindering parameter V t was found to be -0.2600(12) kJ/mol. The value of the parameter V[ 2 describing the top-top coupling in the potential function (via V[ 2 sin3r! sin3i 2 ), was determined to -0.4240(6) kJ/mol.

We present hardware and software details of the pulsed molecular beam microwave Fourier transform (MB-MWFT) spectrometer used in the Kiel microwave group. We emphasize an automatic scanning facility which greatly increases the efficiency of MB-MWFT spectroscopy for the measurement of unassigned spectra.

The microwave rotational spectrum of toluene has been investigated in the region from 8 to 23 GHz. The three types of absorption lines to be expected in the case of a molecule of this symmetry with a very low sixfold barrier hindering internal rotation of the methyl top have been found: to=0; | to | 0, 3 n; |m| = 3n. From the lines to=0 the rotational constants and the dipole moment (0.375 ± 0.01 D) have been determined; from the lines |TO| = 3 the internal rotation barrier could be calculated as V6= (13.94 ± 0.1) cal/mole. This value is discussed and compared with the very much larger value derived from solid-phase heat capacity data.

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