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.

Harmony et. al. a recently published some design specifications for a smaller version of a FTMW spectrometer. In this work they used a perpendicular nozzle arrangement and found that even though the size of the vacuum chamber and Fabry-Perot cavity mirrors had been greatly reduced, the overall sensitivity was nearly the same as a conventional sized resonator. In an effort to establish FTMW spectroscopy as a viable new technique for analytical chemists, we have constructed a spectrometer of similar size for use as an analytical instrument. The vacuum chamber of the instrument is based on a multi-port 12" sphere. An integral end-flange mirror permits a coaxial nozzle arrangement which greatly improves the sensitivity. 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 sec. The entire ! spectrometer is mounted on a mob ile cart for transporting to other laboratories. The per-pulse sensitivity of this smaller instrument is about a factor of 2-3 less than a conventional sized instrument, however the smaller vacuum chamber allows the nozzle to be pulsed much faster without overloading the vacuum pumps. These two factors offset so that the ultimate sensitivity (given one to two minutes of averaging) is approximately the same.

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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.

Polycyclic aromatic hydrocarbons (PAHs) have long been postulated as constituents of the interstellar gas and circumstellar disks. Observational infrared emission spectra have been plausibly interpreted in support of this hypothesis, but the small (or zero) dipole moments of planar, unsubstituted PAHs preclude their definitive radio astronomical identification. Polar PAHs, such as corannulene, thus represent important targets for radio astronomy because they offer the possibilities of confirming the existence of PAHs in space and revealing new insight into the chemistry of the interstellar medium. Toward this objective, the high-resolution rotational spectrum of corannulene has been obtained by Fourier transform microwave spectroscopy, and the dipole moment (2.07 D) of this exceptionally polar PAH has been measured by exploiting the Stark effect.

Sweet truth: The search for sugars in interstellar space is hampered by a lack of spectroscopic information. D‐Ribose is now the first C5 sugar observed in the gas phase using microwave spectroscopy. The rotational spectrum revealed six conformations of free ribose, adopting preferentially β‐pyranose rings and higher‐energy α‐pyranose forms. No evidence of α‐/β‐furanoses or linear forms was found, unlike biological systems, where β‐furanoses are found in RNA.

The microwave spectra of (CH(3))(3)(74)Ge(35)Cl and its isotopologues (CH(3))(3)(72)Ge(35)Cl and (CH(3))(3)(74)Ge(37)Cl have been studied in the frequency range from 3-24 GHz revealing the complex internal dynamics of this organometallic molecule with three internal rotors. The assignment of the complex spectrum has been facilitated by permutation-inversion theory and ab initio calculations. The V(3) barrier to internal rotation is determined to be 372.359(47) cm(-1). Furthermore, an analysis of the chlorine quadrupole coupling yields the description of the Ge-Cl bonding character which is estimated to be dominated by covalent contributions (46.5%) together with 37.6% ionic and 15.9% pi-bonding character. From isotopic substitution the Ge-Cl bond distance could be determined as 2.15198(97) Angstroms.

Two structures of neutral leucine are detected in the jet-cooled rotational spectrum of a laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) experiment. The comparison between the experimental rotational and (14)N nuclear quadrupole coupling constants and those calculated ab initio provides conclusive evidence for the identification of the conformers. The most stable species is stabilized by a N-H...O=C intramolecular hydrogen bond and a cis-COOH interaction, while a higher-energy conformer exhibits a N...H-O intramolecular hydrogen bond and trans-COOH, as in lower aliphatic amino acids. The isobutyl side chain adopts the same configuration in the two conformers of leucine, characterized by a trans arrangement of the C'-C(alpha)-C(beta)-C(gamma)-C(delta) chain. The differences with the preferred side chain configurations observed in valine and isoleucine are discussed.