A portable, pulsed-molecular-beam, Fourier-transform microwave spectrometer designed for chemical analysis

Abstract

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.

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm −1 of internal energy measured by dynamic rotational spectroscopy

“…The gas sample was expanded into vacuum by using a pulsed valve (Series 9, 10 Hz, 0.8-mm orifice; General Valve). The FTMW spectrometer used in these measurements is a compact Balle-Flygare-type instrument based on the recent National Institute of Standards and Technology design (35).…”

Fourier Transform Microwave Spectroscopy Measurement and Instrumentation

“…The linewidth (HWHM) of the individual components is on the order of 1.5 kHz, or 0.00000005 cm −1 ; at an appreciable S/N ratio a frequency accuracy of 150 Hz, or 0.000000005 cm −1 is achieved for unblended lines. The extraordinary resolution provides supersonic-jet expansion FT-MW spectroscopy with a chemical specificity unrivaled by any other analytical method (Suenram et al 1999, Bousquet et al 2005. Furthermore, the sensitivity allows for the observation of, e.g., mono-deuterated asymmetric-top molecules in natural abundance (Dreizler et al 1998).…”

“…While alternative cylindrical cavity designs, utilizing the low-loss TE 01 -mode, have also been tried (Storm et al 1996), considerable improvements were achieved mostly by optimizing the parameters of the Fabry-Pérot-type resonator. Quite compact resonators (Suenram et al 1999) can successfully be employed in the cm wave range. Further developments include cryogenic operation of the spectrometer using a thermally isolated Fabry-Pérot-type resonator .…”

Multidimensional Large‐Amplitude Motion: Revealing Concurrent Tunneling Pathways in Molecules with Several Internal Rotors

“…J transitions with J = 1,…,5 of the rotational spectrum of (CH 3 ) 3 GeCl were recorded at the NIST by using two FTMW spectrometers with pulsed molecular beams. [25][26][27][28] With argon as the carrier gas, the uncertainty in the transition frequency is about 4 kHz. To determine the J + 1 !…”