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.

We report on first experiences with a pulsed molecular beam microwave Fourier transform spectrometer with parallel molecular beam and resonator axes. This setup shows up a high resolution and sensitivity.

M. D. Harmony, K. A. Beran, D. M. Angst, and K. L. Ratzlaff [Rev. Sci. Instrum. 66, 5196 (1995)] recently published some design specifications for a smaller version of a Fourier transform microwave (FTMW) spectrometer. In that work they used a nozzle arrangement which pulsed the molecular beam perpendicular to the axis of the Fabry–Pérot cavity. They found that even though the size of the vacuum chamber and Fabry–Pérot cavity mirrors had been reduced, the overall sensitivity of the instrument was nearly the same as one with a conventional sized resonator. In an effort to establish FTMW spectroscopy as a viable new technique for analytical chemists, we have constructed a miniaturized version of our laboratory instrument for use as an analytical instrument. The vacuum chamber of the instrument is based on a commercially available, multiport 30 cm (12 in.) sphere. An integral end-flange mirror permits a coaxial nozzle injection of the molecular beam which greatly improves the sensitivity of the instrument. The movable cavity mirror rides on a fast motorized stage which allows tuning to any frequency within the range of the spectrometer in 1–2 s. The entire spectrometer is mounted on a mobile cart, allowing it to be easily transported to other laboratories or remote locations. The per-pulse sensitivity of this smaller instrument is slightly less than the larger laboratory instruments, however the smaller vacuum chamber allows the nozzle to be pulsed much faster without overloading the vacuum pumps. The new miniaturized FTMW spectrometer is only a factor of 2 less sensitive than the larger laboratory instrument. This instrument provides analytical chemists with a new tool that can unambiguously identify trace amounts of large organic compounds in gas streams. The instrument also permits real-time analysis which should be useful for monitoring and optimization of process gas streams. Lower detection limits are typically in the nanomol/mol regime.

A number of low-J rotational transitions have been measured for two conformations of the simplest amino acid, glycine, providing precise hyperfine transition frequencies and electric dipole moments for radioastronomy searches for glycine in interstellar clouds. Measurements were made between 12 and 25 GHz with a pulsed-nozzle, Fabry-Perot-cavity, Fourier transform microwave spectrometer equipped with either a heated nozzle or laser ablation-nozzle for vaporization of solid phase glycine. Nuclear quadrupole coupling constants were determined for both conformers from measurements of the hyperfine-split rotational transitions. The quadrupole coupling constants for conformer I are eQq aa ϭ Ϫ1.208(9) MHz and eQq bb ϭ Ϫ0.343(8) MHz and for conformer II are eQq aa ϭ 1.773(2) MHz and eQq bb ϭ Ϫ3.194(4) MHz. The dipole moment components of both conformers were determined to higher precision. We obtain a ϭ 3.039(10) ϫ 10 Ϫ30 C m [0.911(3) D] and b ϭ 2.025(17) ϫ 10 Ϫ30 C m [0.607(5) D] for conformer I, and a ϭ 17.92(11) ϫ 10 Ϫ30 C m [5.372(34) D] and b ϭ 3.10(3) ϫ 10 Ϫ30 C m [0.93(1) D] for conformer II, where the uncertainties shown are 1 standard deviation.

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.