Design, construction, and testing of a large-cavity, 1-10 GHz Flygare-Balle spectrometer

A large pulsed-beam, Fourier transform microwave spectrometer employing 48 in. diameter mirrors and 35(") (NHS-35) diffusion pump has been constructed at the University of Arizona. The Fabry-Perot-type cavity, using the large mirrors provides Q-values in the 15,000 to 40,000 range. Test spectra were obtained using transverse and coaxial injection of the pulsed-nozzle molecular beams. The measured molecular resonance linewidths were 8 kHz for the transverse injection and 2 kHz for coaxial molecular beam injection. Good signal to noise ratios were obtained for the test signals. Strong lines for butadiene iron tricarbonyl were seen with a single beam pulse (S/N = 5/1). Transitions were measured as low as 900 MHz and some previously unresolved hyperfine structure is now resolved for the butadiene iron tricarbonyl spectra. The spectrometer is operated using a personal computer with LABVIEW programs, with provisions for automatic frequency scanning. The extended, low-frequency range of this spectrometer should make it very useful for making measurements on significantly larger molecules and complexes than have been previously studied. The improved resolution, in the coaxial beam mode, will allow better resolution of hyperfine structure. The large diffusion pump allows a higher beam pulse frequency to compensate for the generally lower sensitivity at lower frequencies.